Extension Grant support for clients

[ghstahl]

token-exchange
I have a token exchange implementation that conforms to the specification. For this discussion the response conforms.

I would like to use the client in this library to retrieve the token.
I copied the following example
client-credentials client

The code references internal helpers.

	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, v, internal.AuthStyle(c.conf.AuthStyle))

Can we consider opening these helpers up for clients?

This is an example of the data being sent.

grant_type:urn:ietf:params:oauth:grant-type:token-exchange
subject_token:{{access_token}}
subject_token_type:access_token
requested_token_type:urn:ietf:params:oauth:token-exchange:org
//org_id:430f0845-9289-4b13-8760-111d1e96e03b
scope:offline_access
//org_id:junk

The org_id and scope are optional for my implementation.

I copied the entire internal folder into my project and it works without change when I use it to make a call to my custom token_exchange.