Add option to submit client_id & client_secret via Authorization header to token endpoint

[rcbjBlueMars]

No description provided.

[bigfleet]

@rcbjBlueMars Confirming that you are expecting something of the form:

curl  
-H 'Authorization: Basic 0xFeeDdEAdBeefDeadBeEf' 
--request POST http://host:port/token 
-d '...'

... where the basic header is base64encoded(client_id:client_secret).

The base64 encoding would prevent a 'debugging' of the original client values separately, but could obviously include the encoded content header.

[rcbjBlueMars]

I believe that is what we want, yes.