fix(core): single value enums (const) are not generated correctly in 3.1 specs

[joscha]

This pull request extends the OpenApiNormalizer to translate single const enums (3.1) into an enum with a single value (3.0).
The fix and a unit test is in 9d36e99
The diff of the samples produced by that fix can be seen in 2760c37

Background:

Currently single value enums in 3.1 are not generated correctly.

In 3.1 these two schemas should produce equivalent code:

      SingleValueEnum_3_1:
        required:
          - type      
        properties:
          type:
            const: this-is-my-only-value
            type: string
      SingleValueEnum_3_0:
        required:
          - type
        properties:
          type:
            enum:
              - this-is-my-only-value

(the type property of each model can only ever have the value this-is-my-only-value. For 3.1 specs this seems to be widened to string, which is only technically correct.
It is especially visible in the typescript generator, where values are now widened to string instead of a "this-is-my-only-value" as const type. //cc @macjohnny

Real-world example (a spec that was upgraded from 3.0.1 to 3.1.0:

via planet-a-ventures/affinity-node#49

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[joscha]

this should be:

Suggested change

	'type': string;
	'type': 'this-is-my-only-value';

[joscha]

fix in 9d36e99

[joscha]

This is the actual fix that transforms a single const into an enum with one value, so the 3.1 and 3.0 representations are equivalent

[joscha]

this is the fix (1)

[joscha]

and (2)

[macjohnny]

thanks!

[wing328]

thanks for the PR

cc @OpenAPITools/generator-core-team

[joscha]

thanks for the PR

cc @OpenAPITools/generator-core-team

Thank you. Sorry, wasn't aware there was a core team alias, will use that one from now on.

Update: actually, I think this team doesn't exist? But it would be great if it did, and even greater if we had one per generator? We could then update the pull request template as well with the correct mention, which will work as well for additions and removals and people in the respective team can also tailor their notification preferences around it and/or filter emails

[joscha]

Is there anything I can do to help this one along? I am somewhat blocked in planet-a-ventures/affinity-node#49 until this issue has been resolved.

[wing328]

lgtm. thanks for the PR

[wing328]

fyi. samples updated via c8fad42