CLI should be able to authenticate with new casbin-driven auth middleware

[jakedoublev]

Background

With the work for opentdf/platform#383, the CLI now needs to utilize a token within the SDK calls throughout, bringing the work in #63 to working e2e.

Acceptance Criteria

1. auth flow driven by CLI can store a token for later use
2. all Policy CLI commands authenticate their requests through the SDK
3. updated README with any new instructions

[jakedoublev]

Splitting out the work to read in auth and token endpoints from the platform well-known config into a separate issue.