Honor custom_access_token_attributes in client credentials grant flow

[mlr]

Summary

This change supports the custom_access_token_attributes configuration for Client Credentials grant_flow type.

Without this change, the custom configured attributes are not passed along during the token creation for client credentials requests, which leads to an error similar to:

Mysql2::Error - Field 'tenant_id' doesn't have a default value

If a default of null is provided for the field instead, the result will be the column is not populated with the tenant_id provided during the client credentials request.

Other Information

Steps to reproduce:

1. Install doorkeeper following standard instructions for Rails application
2. Configure the initializer with:
   grant_flows %w[client_credentials] and
   custom_access_token_attributes [:tenant_id]
3. Modify the oauth_access_tokens and oauth_access_grants tables with the tenant_id field as described by the initializer comment for custom_access_token_attributes.
4. Create a Doorkeeper::Application and obtain the client id and client secret.
5. Make a credentials request to obtain a token, i.e.

   curl -H 'Content-Type: application/json' -XPOST 'http://localhost:3000/oauth/token' \
      -d '{ "client_id": "[client id]", "client_secret": "[client secret]", "grant_type": "client_credentials", "tenant_id": "1" }'
   

6. Observe the database exception above which happens because the tenant_id field is not passed along during token creation.

[nbulaj]

LGTM 👍

Thanks!