Add tokenCache in custom credentialProviders

[samikshya-db]

Description

• We should be caching the tokens for custom oauth providers
• This is to incorporate caching as mentioned in the internal code audit
• There are 2 options to add tokenCache : one is re-using the persistent token cache being used in refresh flow, another is to extend cachedTokenSource (in-memory). The decision is made by benchmarking both : internal doc.

Testing

• added unit tests

Additional Notes to the Reviewer

[gopalldb]

isn't token exchange happening for every token if host is different? Though the actual token is cached in their own provider, but still exchanged token is not being reused. Can we cache that as well?

[samikshya-db]

CachedTokenSource will cache this exchanged token - atleast till the time it expires. Or are you pointing at something else?

[gopalldb]

Can we do 1 manual testing for each of affected auth mechanisms?

[gopalldb]

Thanks, make sure that you verify these manually