Add support to add optional parameters in OAuth2 introspection request

[ldclakmal]

Description:
Need to support to add optional/custom parameters in OAuth2 introspection request.

Example: Support to add optional parameters like client_id and client_certificate_thumbprint

POST /oauth2/introspect 
HTTP/1.1Host: www.sample.com
Content-Type: application/x-www-form-urlencoded

Authorization: Basic some-token

client_id="some-value"&client_certificate_thumbprint=another-value&token=sample-token&token_type_hint=access_token

https://tools.ietf.org/html/rfc7662#section-2.1

[ldclakmal]

Since those parameters are dynamic values, this cannot be fixed with the existing design. Hence, this will be addressed in future releases.