* Make several params to WorkloadIdentityCredential optional and read them from the environment instead.
* Fix typo in calling the ClientCredentialCore ctor
* Add changelog entry.
* Address PR feedback, avoid creating many WIC options instances.
* Update ctor impl and add options test.
* Set locals if the options are non-empty, and void unused variable in tests.
* Fixup the sample since the customer no longer needs to explicitly pass in those params.
