[FEATURE] Support Token Federation auth type

[bgrams]

Problem Statement
Support OAuth token federation as an official auth type.

Proposed Solution
The current guidance is to perform the exchange using curl, and pass the returned access token to subsequent requests. We would love to see first-class support for this in the SDK along with configuration using env vars. This may be its own auth type (e.g. oauth-federated) or include additional logic within the existing oauth-m2m to conditionally perform the token exchange.

Additionally consider support for loading the subject token from a file to support refreshes from an external process where it may originate (e.g. k8s).