[swift] refactor global fields to local fields in API classes

[kalinjul]

Generated Swift code used global static variables for important state like customHeaders and requestBuilderFactory (see APIs.mustache).
This likely introduces race conditions if one needs to set a custom requestBuilderFactory or header for a single request, as it is not guaranteed that no other Thread/Task re-sets this global var concurrently.

This PR adds local fields in generated Api classes that are initialized with the existing static fields by default.
Api calls are no longer class funcs.

You can now create two instances of an API using different RequestBuilders, headers, and so on. In other platforms such as kotlin or java, this was always possible using different ApiClients.

For backwards compatibility, you may just use the constructor with it's default values, which pulls in the static values just as before.

fixes #17377

TODO: This currently only adresses swift5, swift6 in progress.

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[4brunu]

Hi @kalinjul.
I would like to suggest to you checking the swift 6 generator and consider contributing to it.
Swift 6 already supports different headers, base url, etc, per request, although it still uses the global variables, because the variables were moved into a singleton.
But the swift 6 generator is aligned with what you are trying to archive here.
Please check if it fills yours needs or if still needs some tweaking.