[monitoringlib/auth] Add support for JWTs as client authentification (RFC 7523 section 2.2)

[the-glu]

This PR add support for client authentication as JWTs, implementing RFC 7523 section 2.2.

On top of units tests, this as been tested with a keycloak, configured as such:

with the test auth key (reused as a pair):

leading to successful token retrieval:

uv run python -m monitoring.get_access_token --spec "PrivateKeyJWT(http://localhost:8181/realms/uss/protocol/openid-connect/token, https://uss1.localutm, key_path=build/test-certs/auth2.key)" --audience https://dss.uss1.localutm --scopes "https://dss.uss1.localutm"

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