Do not ignore multiple types when serializing to 3.0

[Youssef1313]

Fixes #2939

• The change to ToSingleIdentifier is only a simplification to make it easier to read.
• Type serialization is now split to two path to be easier to read, especially that 3.0 needs more parameters/info than 2.0.
• The behavior of 2.0 is maintained as-is, no change there
• Previously, in 3.0, we ignored when Type had multiple values (e.g, String and Integer). This is now handled using oneOf or anyOf (whichever isn't used by the current schema already). If both are used, we ignore as it we used to in the past.
• The general idea here is:
  1. We take the Type.
  2. We remove Null from it (if it exists).
  3. If, after removing null, we have a single type, we simply emit it, and conditionally emit nullable: true depending on whether null was existing. This is the same as the existing behavior.
  4. If, after removing null, we have more than one type, we create a oneOf/anyOf with children schemas handling each type individually.
• The reason around all that is that because type only supports a single value when using OpenApi 3.0.

For context: https://spec.openapis.org/oas/v3.0.4.html#json-schema-keywords

The following keywords are taken from the JSON Schema definition but their definitions were adjusted to the OpenAPI Specification.
type - Value MUST be a string. Multiple types via an array are not supported.

[Youssef1313]

Before my change, this was resulting in an empty schema, which will match anything.

[Youssef1313]

Before my change, the type was ignored, and only anyOf was emitted.

[Youssef1313]

Before my change, the type was ignored and only anyOf was emitted.

[Youssef1313]

Before my change, the type was ignored and only oneOf was emitted.

[Youssef1313]

Before my change, only oneOf was emitted.

[Youssef1313]

Before my change, an empty schema was generated, allowing anything to match.

[Youssef1313]

In this test, the behavior is the same as in the past. We are not respecting Type, and I can't see an easy way to respect it.

It's technically possible to emit "allOf" that contains a schema representing the types as "oneOf" or "anyOf" schema, and then another schema that wraps the existing anyOf/oneOf. But that's a major re-write I didn't want to introduce.

[Youssef1313]

Same here. This wasn't (and is still not) respected. We could respect it but it will be a more bigger re-write.

[baywet]

I believe this kind of normalization should not be done during serialization, but rather through a walker which would be called before serialization.

See #808 and other related issues.

Case in point, we've had to revert some of the normalization done for null/nullable (to oneOf) in #2933