[Doc] ext_param_client_id and ext_param_client_secret in DCR request

[nuwandiw]

Ability is provided to register OAuth application with provided client ID and secret value.

[1] should contain following parameter details
ext_param_client_id (optional) schema - string
ext_param_client_secret (optional) schema - string

[2] should contain following.
Sample Request
curl -k -X POST -H "Authorization: Basic YWRtaW46YWRtaW4=" -H "Content-Type: application/json" -d '{"client_name": "application_test","grant_types": ["password"], "ext_param_client_id":"provided_client_id0001", "ext_param_client_secret":"provided_client_secret0001" }' "https://localhost:9443/api/identity/oauth2/dcr/v1.0/register"

Sample Response
{"client_name":"application_test","client_id":"provided_client_id0001","client_secret":"provided_client_secret0001","redirect_uris":[""]}

[1] https://docs.wso2.com/display/IS560/apidocs/OAuth2-dynamic-client-registration/index.html#!/models#RegistrationRequest
[2] https://docs.wso2.com/display/IS560/apidocs/OAuth2-dynamic-client-registration/index.html#!/operations#OAuthDCR#registerApplication

[nuwandiw]

Related to wso2/product-is#3406