CamundaCloudTokenProvider - Allow disabling of file system credentials cache persistence and add tolerance for token expiration

[foliv57]

• Introduces a DisableCredentialsCachePersistence() builder method in client CamundaCloudClientBuilder to prevent persisting token cache in file system. The goal is to allow users to setup an in-memory only cache for e.g. serverless usage of the client.
• Fixes concurrent access issue to AccessToken cache. The introduction of a SemaphoreSlim will protect concurrent accesses to the dictionary and the file system, and prevent concurrent token fetching. The choice of SemaphoreSlim is done because it is the lock mechanism that fits the best with async/await model. The implementation is optimized to prevent locking when a valid token is in cache.
• Introduces a UseAccessTokenDueDateTolerance(TimeSpan tolerance) builder method in client CamundaCloudClientBuilder. The goal of this option is to allow the user to refresh the AccessToken slightly before expiration to prevent that, because of network latency, a token considered valid by the client is expired when validated by the authorization server.

This change also solves partially issue 637. But the issue rather mentions introducing different implementation of the IAccessTokenCache. This approach is more impacting for the PersistedAccessTokenCache for just an in-memory only alternative, as this implementation already have its in-memory cache. To limit the impact, I chose to only improve the existing implementation.

closes #415 #642

[CLAassistant]

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