feat: add support for universe domain

[bshaffer]

• CL-R1 Client Libraries must allow the end-user to configure the endpoint override alongside the universe
• CL-R2 Client Libraries must use the universe_domain variable/property/method name
• CL-R3 Client Libraries must use the universe_domain to form the service endpoint and should deprecate the hardcoded endpoint values
• CL-R5 Client Libraries must use the universe_domain from the credentials to verify that the credentials can be used with the configured universe domain
  • CL-R5.1 If the end-user has not configured the universe_domain explicitly, the Client Libraries must use GDU as the configured universe domain.
  • CL-R5.2 The client libraries must not use the endpoint to validate the universe domain
• CL-R6 Client Libraries should surface either the credentials or the universe_domain to the end-user
  • this is not currently supported (there are no "getters" on the client classes). I personally don't want to add support
  • CL-R6.1 Client Libraries should surface the service endpoint to the end-user
    • this is not currently supported (there are no "getters" on the client classes). I personally don't want to add support for it unless it's a hard requirement
• CL-R7 Client Libraries must allow the end-user to configure the universe domain explicitly
• CL-R8 Client Libraries must update the documentation if they allow the end-user to pass the authentication token/JWT instead of credentials - N/A
• CL-R9 Libraries must avoid leaking the authentication tokens and user data to the GDU
• CL-R10: Configuring mTLS with a universe domain other than GDU must throw an exception

[bshaffer]

Dang, I tried renaming the branch to keep the PR open, but github still closed it :-/

[vishwarajanand]

Lgtm