Bug#37309796 - Inconsistent authentication parameter format and names for MRS Authentication

Description
===========
As an MRS user, I expect that the login workflow for different authentication apps should be,
as much as possible, similar with regards to the required authentication parameter names,
values and mechanisms with which these are specified.

Currently different workflows for different authentication apps also use different ways of
specifying the same authentication parameters and use a confusing mix of both query parameters
and JSON request bodies.
Additionally, in certain cases, the same parameters are encoded using different names for different
workflows, which can also be confusing and potentially avoided (e.g. "user" vs "username", "app" vs
"authApp").

Fix
===
The following AUTH-APPs will use "username" JSON field for transferring user information in POST requests:

* MRS
* MySQL Internal

(POST-PUSH FIX WL#15440)

Change-Id: I5eb238baa1cc0f4e313d655a9dd8cb0735d4b720

Author: lkotula

router/src/mysql_rest_service/src/mrs/authentication/helper/scram.cc

@@ -191,7 +191,7 @@ class JsonAuthInitRequest
  public:
   bool String(const Ch *cstr, rapidjson::SizeType clength, bool) override {
     if (!is_object_path()) return true;
-    if ("user" == get_current_key())
+    if ("username" == get_current_key())
       result_.user.assign(cstr, clength);
     else if ("nonce" == get_current_key())
       result_.nonce.assign(cstr, clength);

router/tests/mrs_client/client/authentication.cc

@@ -491,7 +491,7 @@ Result Authentication::do_scram_post_flow(
 
   JsonObject request_data{
       {"sessionType", (st == SessionType::kJWT ? "bearer" : "cookie")},
-      {"user", user},
+      {"username", user},
       {"nonce", scram.initial_nonce_}};
 
   if (auth_app.has_value()) {