[PM-20592] [PM-22737] [PM-22738] Send grant validator (#6151)

**feat**: create `SendGrantValidator` and initial `SendPasswordValidator` for Send access grants  
**feat**: add feature flag to toggle Send grant validation logic  
**feat**: add Send client to Identity and update `ApiClient` to generic `Client`  
**feat**: register Send services in DI pipeline  
**feat**: add claims management support to `ProfileService`  
**feat**: distinguish between invalid grant and invalid request in `SendAccessGrantValidator`

**fix**: update parsing of `send_id` from request  
**fix**: add early return when feature flag is disabled  
**fix**: rename and organize Send access scope and grant type  
**fix**: dotnet format

**test**: add unit and integration tests for `SendGrantValidator`  
**test**: update OpenID configuration and API resource claims

**doc**: move documentation to interfaces and update inline comments  

**chore**: add TODO for future support of `CustomGrantTypes`

test/Identity.IntegrationTest/openid-configuration.json

@@ -15,6 +15,7 @@
     "api.installation",
     "internal",
     "api.secrets",
+    "api.send.access",
     "offline_access"
   ],
   "claims_supported": [
@@ -33,6 +34,7 @@
     "providerserviceuser",
     "accesssecretsmanager",
     "sub",
+    "send_id",
     "organization"
   ],
   "grant_types_supported": [
@@ -43,7 +45,8 @@
     "password",
     "urn:ietf:params:oauth:grant-type:device_code",
     "urn:openid:params:grant-type:ciba",
-    "webauthn"
+    "webauthn",
+    "send_access"
   ],
   "response_types_supported": [
     "code",