Support normalizing anyof/oneof enum constraints to a single enum

[wing328]

based on #21893 with updated samples, fixed python tests and fixed unit tests (case sensitive file names)

Fixes #15701

This changes the OpenAPINormalizer: If you specify a oneOf or an anyOf where the schema and subschemas have the no or the same type, and all contain just enums, it will change the schema to an enum schema.

this is to support the following format:

test_enum:
  type: string
  oneOf:
    - enum: [TEST1]
      description: testing 1
    - enum: [TEST2]
      description: testing 2
    - enum: [TEST3]
      description: testing 3

Which it now normalizes into:

test_enum:
  type: string
  enum: [TEST1, TEST2, TEST3]

Which is a relatively common workaround-pattern to support the lack of descriptive labels for enum values.
It supports both anyOf and oneOf, and works with numbers, booleans and strings.
It does not keep the title or description in the model. Doing that would probably require moving this from the normalizer to the model or code generator, as openAPI as far as I know has no way of keeping these titles in enum constants.
I made this behaviour configurable with the SIMPLIFY_ONEOF_ANYOF_ENUM configuration option. I enabled it by default

To check, see the automated tests I added, and the example I added to the documentation.

Potential discussion topics:

• I am not entirely sure if at some places I should also check instanceof StringSchema, or write a utility function that gets the type from the type of schema, instead of just calling ModelUtils.getType().
• should this behaviour be enabled by default?
• Since I made this configurable, should this be against master or 8.0.x?

Potential additions (not within my current scope):

• there is already processSimplifyAnyOfStringAndEnumString, which does something similar, but only works with two schemas, where one is a string and one an enum. By also recognizing strings, this new normalization feature could also replace this functionality in a more generic way. But that would break compatibility of the old configuration aption
• recognizing string or integer constants as part of an enum would be a nice addition - that would also support oneOf with several constants as generating an enum

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[pekuz]

Seemingly a detail but it helps a lot. Thank you @wing328.