* Further Elixir Client Improvements

Resolves #12731

 and is the completion of the work that I started with
#12751.

The changes here are extensive and likely resolve an issue that I have
seen with the Ory SDK (ory/sdk#194). I have also been unable to run the
integration suite for Elixir as I am (trying) to run everything in
Docker (`./run-in-docker.sh`) as I *do not* have a suitable Java
development environment set up, and do not do enough Java work to really
justify it.

- Updated the README for Elixir projects. Aside from some improved
  readability of the template by use of link references instead of
  inline links, I have also fixed the examples:

  - The `deps` example should have been putting a version constraint
    related to `appVersion`.

  - The `config` example should have been using `packageName` instead of
    `appName`. This particular issue repeats.

- In all Elixir files:

  - Changed the function `@docs` formatting:

    - changed the ehading level for `Parameters` and `Returns` to h3
      (`###` instead of `##`). This will make somewhat better looking
      documentation that does not over-emphasize these details (which
      are *not* documented in a normal Elixir way, but this is somewhat
      to be expected with a code generator.) It may be desirable, after
      testing, to change this to `h4` instead of `h3`.

    - Put parameter names and most return types in in-line code blocks
      (`` `hello` ``).

    - Put return types, when there are multiple types, in a Markdown
      list.

  - Fixed a lot of the spacing. Most files will be *closer* to Elixir
    standard formatting than they were. Because of the limitations of
    Mustache, it is still recommended that people who generate Elixir
    clients run `mix format` at least once on their codebase.

- `api.mustache`:

  - Removed an awkward function pipeline call. If we specified at least
    Elixir 1.12 (something that I do not recommend as we have recently
    jumped from requiring Elixir 1.6 to Elixir 1.10), there is a better
    way to specify this now with `Kernel.then/2`. In the meantime,
    assigning the constructed request structure to a variable and then
    making a separate pipeline for the request execution and handling
    makes for *much* easier to read generated code.

  - Fixed the extra space issue with `evaluate_response` call tuple
    values; `{{=<% %>=}}` changes the tag types, so this change is
    intentional.

- In `config.exs.mustache`, `runtime.exs.mustache`, `mix.exs.mustache`,
  and `connection.ex.mustache`, use `packageName` instead of `appName`
  for configuration specification. If `packageName` and `appName`
  differed, we would end up with cases like ory/sdk#194.

- `connection.ex.mustache` has been almost entirely rewritten. The
  changes started in order to eliminate a `@doc` compile-time warning,
  but shifted to remove the old way of building Tesla client structs
  with `use Tesla`. It works, but is no longer the recommended way of
  building Tesla clients.

  - The *recommended* way of building a Tesla Client would now be
    `Tesla.client(Connection.middleware(), Connection.adapter())`.

  - Exposed both `Connection.adapter/0` and `Connection.middleware/1`
    for use. `Connection.middleware/1` has special handling for the
    cases where OAuth2 or HTTP Basic Auth are defined in the
    application, but do not currently handle any other auth methods.

- `deserializer.ex.mustache` has mostly been reformatted. There are
  things that I do not like about it (I do not like pipelines with one
  line), and I have expanded one function capture into an anonymous
  function for readability.

- `request_builder.ex.mustache` has been updated with better
  function and parameter descriptions and names. Please note that if
  `request |> method(:delete) |> method(:post)` is supposed to produce
  a `POST` operation, we will need to change from `Map.put_new/3` to
  `Map.put/3`.

  - Reordered `evaluate_response/2` so that it is the function documented,
    and made `decode/2` and `response_mapping/3` private functions. As
    far as I can tell, I have *not* changed the functionality.

* Address issues found in code review

- The example dependency code in the README had dropped the opening
  brace for the tuple. This has been resolved.

- The default formatting of the API pipelines has been adjusted to
  minimize possible changes from `mix format`.

* Update modules/openapi-generator/src/main/resources/elixir/api.mustache

Co-authored-by: Michael Ramstein <633688+mrmstn@users.noreply.github.com>

* Update modules/openapi-generator/src/main/resources/elixir/connection.ex.mustache

Co-authored-by: Michael Ramstein <633688+mrmstn@users.noreply.github.com>

* Update modules/openapi-generator/src/main/resources/elixir/connection.ex.mustache

Co-authored-by: Michael Ramstein <633688+mrmstn@users.noreply.github.com>

* Update templates based on review comments

Co-authored-by: Michael Ramstein <633688+mrmstn@users.noreply.github.com>