[WIP] typescript-axios: support data/date-time to Date conversion

Created by: tibbe

WIP discussion starter.

I would like to judge the interest of adding an opt-in flag for having the typescript-axios generator support date/date-time conversion (and in the future perhaps full schema validation of the server response), like the typescript-fetch generator does today. The implementation would be (very) similar, with generated MyModelFromJSON functions and a isDate[Time]Type check to only enabled this if the user explicitly maps date/date-time to Date (string being the current default).

Is there interest in having this? It's quite a bit of work so I'd like to have some idea before going further. The existing commit only implements perhaps 20% of what's needed. I would probably need some guidance in what parts of typescript-fetch needs to be copied (e.g. typescript-fetch has a large ExtendedCodegenParameter class that I'm not sure is needed to just support e.g. isDateType).

Here's what I think are the remaining TODOs in this PR:

• Add isDate[Time]Type to only enable this behavior if date/date-time are explicitly mapped to Date (default: off).
• Add MyModelFromJSON calls in the right places, including dealing with arrays/maps.
• See if anything can be shared between typescript-axios/typescript-fetch (I don't know if sharing between generators is encouraged).

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When date/date-time formats are mapped to Date objects the Axios JSON payload is converted to a JavaScript Date object before it is returned.

Does not change the default behavior, as date/date-time is mapped to string by default in the typescript-axios generator.

Fixes #13188.

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