Add Support for client_credentials Auto Configuration

[rwinch]

With the changes in f5deebf#diff-b716836901d5fea39e485e35997f2f62 the configuration below works but longer makes sense:

spring:
  security:
    oauth2:
      client:
        provider:
          idp:
            issuer-uri: https://example.com/
        registration:
          authorizationcode:
            idp:
              client-id: client-id
              client-secret: client-secret
              authorization_grant_type: client_credentials

Notice that a client_credentials grant type is being configured under the authorizationcode parent.

Related #14554

[mbhave]

Closing in favor of #14625.