add "from oidc discovery endpoint" to add ext jwt

[dovholuknf]

Multiple fields from the oidc discovery endpoint are required when creating an external jwt signer. It would just be easier to pull these values from the oidc discovery endpoint.

If possible, it'd be nice to have a button that was like "get most of this data from the oidc discovery endpoint".

It should set:

• issuer from issuer
• jws endpoint from jwks_uri
• external auth url from the discovery endpoint, without .well-known/openid-configuration. So if the discovery endpoint is at: https://my.idp/.well-known/openid-configuration, this value would be https://my.idp

this is not a high priority, but it'll help fill the form out.