Support RFC 7523 JWT tokens for OAUTH client authentication

*Title*: Support RFC 7523 JWT tokens for OAUTH client authentication

*Description*:
[RFC 7523 describes a way for applications to avoid long-lived credentials in the form of client ID/secret by using a JWT token](https://datatracker.ietf.org/doc/html/rfc7523#section-2.2):

> To use a JWT Bearer Token for client authentication, the client uses the following parameter values and encodings.
> The value of the "client_assertion_type" is "urn:ietf:params:oauth:client-assertion-type:jwt-bearer".
> The value of the "client_assertion" parameter contains a single JWT. It MUST NOT contain more than one JWT.

Concretely, this would allow using [Kubernetes service account tokens](https://kubernetes.io/docs/concepts/storage/projected-volumes/#serviceaccounttoken) to authenticate with an IdP.