Add support for additional params on access_url

[marque88]

The request is to add additional config param similar to custom_params (something like custom_token_form_params) to add additional params in the POST request payload

[simov]

What parameters do you want to send? Also, which provider is this? I'm asking because it's a bit uncommon to send custom parameters with that request compared to the authorization URL, for example. Usually custom parameters sent to the token endpoint are handled internally by Grant, but a few common options are exposed as configuration as well.

[marque88]

Hi @simov , I want to send identityDomain parameter that identify the tenant into Oracle Access Management.
The provider is a private company that use this product to support Oauth2 OpenId Connect protocol.

For now I patch your plugin adding into your oauth2.js file my custom provider and add there the param

[unrevised6419]

I'm not sure, but this might be related to #317