CBST2-06: Implement rate limiting for JWT auth failures

[jclapis]

This adds a customizable setup to the signer module for rate limiting requests after repeated JWT authorization failures from a client. The implementation is simple: if a client fails JWT authorization X times, they are added to a cooldown and any subsequent requests will be denied until the cooldown completes.

The failure limit is controlled in the [signer] config with jwt_auth_fail_limit, or with the SIGNER_JWT_AUTH_FAIL_LIMIT_ENV environment variable.

The cooldown timeout (in seconds) is controlled in the [signer] config with jwt_auth_fail_timeout_seconds, or with the SIGNER_JWT_AUTH_FAIL_TIMEOUT_SECONDS_ENV environment variable.

As a side-effect, this also adds a unit testing setup for the Signer module since one wasn't in place yet. A test for this rate limiting behavior is included.