Fix issue with fastapi generator converting all fields to snake_case

[RoryDungan]

I noticed that the Python-FastAPI generator converts all model field names to snake_case to match Python convention. While this is desireable from a code-style perspective, it breaks any API using a different convention for field names since it also affects the names of fields that get serialised to JSON.

FastAPI uses Pydantic for its model classes, which supports setting an alias value in order to deserialise a different name to what is used for the Python class, so I modified the generator to use this. Be default it will always be used during deserialisation but only during serialisation if the by_alias=True option is specified. Luckily, FastAPI includes an option we can set on endpoints that will use this for data returned by the endpoint, response_model_by_alias=True.

I've enabled this everywhere because there's no harm in using it if the alias is the same as the name and it seemed like adding logic to specifically detect camelCase field names would overcomplicate things, plus this could potentially be useful if you have an OpenAPI spec that includes a field name that is a reserved word in Python, although I haven't tested it in that case.

Here's an example of some code before and after the change:

# before:
class RenderClip(BaseModel):
    """NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).

    Do not edit the class manually.

    RenderClip - a model defined in OpenAPI

        focused_player_actor_id: The focused_player_actor_id of this RenderClip.
        time_range: The time_range of this RenderClip.
        ingredients: The ingredients of this RenderClip.
        output_length: The output_length of this RenderClip [Optional].
    """

    focused_player_actor_id: str
    time_range: GameTimeRange
    ingredients: List[Ingredient]
    output_length: Optional[float] = None

# after:
class RenderClip(BaseModel):
    """NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).

    Do not edit the class manually.

    RenderClip - a model defined in OpenAPI

        focused_player_actor_id: The focused_player_actor_id of this RenderClip.
        time_range: The time_range of this RenderClip.
        ingredients: The ingredients of this RenderClip.
        output_length: The output_length of this RenderClip [Optional].
    """

    focused_player_actor_id: str = Field(alias="focusedPlayerActorId")
    time_range: GameTimeRange = Field(alias="timeRange")
    ingredients: List[Ingredient] = Field(alias="ingredients")
    output_length: Optional[float] = Field(alias="outputLength", default=None)

This may also fix #11604 since it allows serializing/deserializing models with names in camelCase without requiring any additional options in the generator.

I've made this change on top of 5.4.x because that's what our team is using so if this could be included in a patch release of 5.4.x that would be ideal, although I have also tested on master and it works so if you think it would be more appropriate to commit there I'm happy to rebase.

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  This change applies to the Python-FastAPI server generator @taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @arun-nalla @spacether

[RoryDungan]

The build failed with this error:

Cannot run program "gradle" (in directory "/home/circleci/OpenAPITools/openapi-generator/samples/client/petstore/groovy"): error=2, No such file or directory

I can't see how this is related to my change, so maybe it's an existing flakiness or an existing issue on the 5.4.x branch?

[spacether]

Can you please rebase on master? We don't do later releases of old builds like 5.4.x

[RoryDungan]

@spacether I've rebased on master now

[wing328]

Tested passed via https://app.travis-ci.com/github/OpenAPITools/openapi-generator/builds/250402172