Validate id token on service account token request

[j-zimnowoda]

Is your feature request related to a problem? Please describe.
Currently, there is no identity validation on GET /tokens/{saNme} request
It means that any malicious actor can request SA token, if SA has been already created.

Describe the solution you'd like to see

• add Authorization header that contains ID token for GET /tokens/<sa-name> request,
• validate Authorization header content by validating ID token signature.