[BUG] [Go] Execution responses and errors are values not pointers

Created by: aeneasr

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
Have you searched for related issues/PRs?
What's the actual output vs expected output?
[Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Generating a Go-client yields in code that does not adhere to Golang's error handling. Request executors return GenericOpenAPIError as a value:

func (r AdminApiApiDeleteIdentityRequest) Execute() (*_nethttp.Response, GenericOpenAPIError) {
	return r.ApiService.DeleteIdentityExecute(r)
}

which makes it impossible to check if err != nil as the variable is a value (and thus always not nil) and not a pointer. Instead, this should read:

func (r AdminApiApiDeleteIdentityRequest) Execute() (*_nethttp.Response, *GenericOpenAPIError) {
	return r.ApiService.DeleteIdentityExecute(r)
}

The same applies to return values which should also be pointers, not structs:

func (a *HealthApiService) IsInstanceReadyExecute(r HealthApiApiIsInstanceReadyRequest) (HealthStatus, *_nethttp.Response, GenericOpenAPIError) {

should be:

func (a *HealthApiService) IsInstanceReadyExecute(r *HealthApiApiIsInstanceReadyRequest) (HealthStatus, *_nethttp.Response, *GenericOpenAPIError) {

openapi-generator version

5.0.0

OpenAPI declaration file content or url

https://gist.github.com/aeneasr/3c11c5b454df6c5e4d8e7a3e16d04db7

Generation Details

# gen.go.yml
disallowAdditionalPropertiesIfNotPresent: false
packageName: kratos
generateInterfaces: false
isGoSubmodule: false
structPrefix: true

$ npm run openapi-generator-cli -- generate -i ".schema/api.openapi.json" \
				-g go \
				-o "internal/httpclient" \
				--git-user-id ory \
				--git-repo-id kratos-client-go \
				--git-host github.com \
				-c .schema/openapi/gen.go.yml

Steps to reproduce

See above.

Related issues/PRs

Suggest a fix

I know too little java and internals of how this system works. But basically calls to GenericOpenAPIError{/* ... */} should always be pointers &GenericOpenAPIError{ /* ... */ }. Same for API return values.