Improve CamundaCloudTokenProvider to work with SM

[ChrisKujawa]

Note

This PR is based on #607

Hey @LennartKleymann

first of all thank you very much for your time and the great contribution! 🙇🏼
I had a look at your PR, and since it is coming from our fork I was not able to modify it directly, so I created a new one based on yours.

Short description of what I did and why:

I had a look at your PR and felt that introducing the base class might not be optimal. It felt complex, and there were still some responsibilities lying in the sub-classes (like writing the caches, etc) and we had still some duplications between the classes.

In general, I favor composition over inheritance so I introduced a new interface and implementation for caching an AccessToken. The access token is persisted within the PersistedAccessTokenCache. The cache class has full responsibility, for caching and persisting.

It makes the unit tests, in this case much easier, and also the classes that use the cache.

Furthermore, I realized that our authentication API also supports form-url-encoding (I tried it manually), so I migrated from sending JSON content to form-url-x-encoding. This should help to support Keycloak as well. Since this was the only difference in these classes I removed the newer ones (I couldn't remove the old TokenProvider to not break earlier clients).

Thank you very much for providing the integration tests! 🎉 I adjusted them to use the CamundaCloudTokenProvider, and it worked without any issues. Furthermore, I tried again manually with the changes to run against SaaS, which worked as well.

// At begin...
2023-12-15 15:02:20.6464 | DEBUG | Zeebe.Client.Impl.Builder.CamundaCloudTokenProvider | Received access token for 3ae1aee9-1654-4801-b326-f930acb2322b.chaos-1.zeebe.ultrawombat.com 
// Later
2023-12-15 15:02:21.0047 | DEBUG | Grpc.Net.Client.Internal.GrpcCall | Starting gRPC call. Method type: 'Unary', URI: 'https://3ae1aee9-1654-4801-b326-f930acb2322b.chaos-1.zeebe.ultrawombat.com/gateway_protocol.Gateway/DeployResource'. 
2023-12-15 15:02:21.0052 | TRACE | Zeebe.Client.Impl.Misc.PersistedAccessTokenCache | Use in memory access token 
2023-12-15 15:02:21.0052 | TRACE | Grpc.Net.Client.Balancer.Internal.ConnectionManager | Pick started. 
2023-12-15 15:02:21.0063 | DEBUG | Grpc.Net.Client.Balancer.Internal.ConnectionManager | Successfully picked subchannel id '1-1' with address 3ae1aee9-1654-4801-b326-f930acb2322b.chaos-1.zeebe.ultrawombat.com:443. Transport status: ActiveStream 
2023-12-15 15:02:21.0063 | TRACE | Grpc.Net.Client.Balancer.Internal.BalancerHttpHandler | Sending request https://3ae1aee9-1654-4801-b326-f930acb2322b.chaos-1.zeebe.ultrawombat.com/gateway_protocol.Gateway/DeployResource. 
2023-12-15 15:02:21.0063 | DEBUG | Grpc.Net.Client.Internal.GrpcCall | Sending message. 

closes #503
closes #607