Try to verify access_token as JWT and extract expiry

We try to parse the access token as a JWT now. Verifying the signature
using the keys downloaded from the jwks_uri of the IDP. Currently we
only use it to extract the expiry information from the JWT. This could
be reworked to extract other claims from the token for authorization
purposes.

Fixes: #3841
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f