feat: add auth client and generate_disposable_token api

[anitarua]

Work towards #419

• Implemented AuthClient and builder. No config object since there's no logging object to configure in this SDK like in the JS or Go ones.
• Implemented permissions structure (PermissionScope and DisposableTokenScope and related objects)
• Implemented generate_disposable_token method
• Added docs and doctests for public-facing objects
• Added integration tests to verify correctness of generate_disposable_token implementation
• Updated test-util and on-pull-request workflow to accommodate new tests

After cutting a release, I can fast-follow with updating the docs snippets example file and unit tests for the permissions conversions.

[benbpyle]

Thanks for implementing this feature!

[honeyAcorn]

I've verified the capability by using momento console to generate superuser api key.
Then use the Superuser API key to generate disposable token using rust sdk.
Those disposable token can only be used toward certain cache for only read or write or both.
Everything looks good except for some missing test case.

1. prefix-write
2. Can we add a verify for the expire time? Add one case to expire in a few seconds and test if it expires in integration test.

[honeyAcorn]

LGTM!