Add 2 rules to OpenAPI Normalizer

add 2 rules to openapi normalizer:

• REMOVE_ANYOF_ONEOF_AND_KEEP_PROPERTIES_ONLY: when set to true, oneOf/anyOf schema with only required properies only in a schema with properties will be removed. (example)

• SIMPLIFY_ANYOF_STRING_AND_ENUM_STRING: when set to true, simplify anyOf schema with string and enum of string to just string

FYI @OpenAPITools/generator-core-team

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