osbuild-mock-openid-provider: support client_credentials grant type

Extend the implementation of mock openid server to take the `grant_type`
into consideration for the `/token` endpoint.

In addition to the previously supported `refresh_topen`, the
implementation now supports also `client_credentials`.

This is necessary to make it possible to use the mock server in
the `koji-osbuild` CI, because the builder plugin uses
`client_credentials` to get access token.

The implementation behaves in the following way:
 - For `refresh_token` grant type, it takes the `refresh_token` value
   from the request and adds it to the `rh-org-id` field in the custom
   claim, which is part of the returned token.
 - For `client_credentials` grant type, it takes the `client_secret`
   value from the request and adds it to the `rh-org-id` field in the
   custom claim, which is part of the returned token.

Requests without the supported `grant_type` set are rejected.

Modify affected test cases to specify `grant_type` when fetching a new
access token.

test/cases/api/common/common.sh

@@ -60,6 +60,7 @@ function access_token {
 function access_token_with_org_id {
   local refresh_token="$1"
   curl --request POST \
+    --data "grant_type=refresh_token" \
     --data "refresh_token=$refresh_token" \
     --header "Content-Type: application/x-www-form-urlencoded" \
     --silent \