Consider creating `ClientAssertionCredential` and implementing `WorkloadIdentityCredential` in terms of it

[ahsonkhan]

Other languages like .NET and GoLang have this credential. We can start with this being private implementation detail of WIC and turn it to be public once there is a customer need for it in C++.

https://github.com/Azure/azure-sdk-for-net/blob/a932f53e2fc200cee443614a0bf991b02b518055/sdk/identity/Azure.Identity/src/Credentials/ClientAssertionCredential.cs#L18
https://github.com/Azure/azure-sdk-for-go/blob/e7968b534c61da1e38a683e21ac690deb14833c7/sdk/azidentity/client_assertion_credential.go#L26

WIC composing CAC:
https://github.com/Azure/azure-sdk-for-go/blob/e7968b534c61da1e38a683e21ac690deb14833c7/sdk/azidentity/workload_identity.go#L28

cc @chlowell

[ahsonkhan]

This is needed for #4968

[sershe-ms]

Hmm, since #4968 is closed, is it possible to expose this class for compatibility with the C# SDK?