test: add load test for session creation (#8088)

# Which Problems Are Solved

Extends load tests by testing session creation.

# How the Problems Are Solved

The test creates a session including a check for user id.

# Additional Context

- part of https://github.com/zitadel/zitadel/issues/7639

load-test/README.md

@@ -36,6 +36,9 @@ Before you run the tests you need an initialized user. The tests don't implement
 * `make machine_pat_login`  
   setup: creates machines and a pat for each machine  
   test: calls user info endpoint with the given pats
+* `make machine_client_credentials_login`  
+  setup: creates machines and a client credential secret for each machine  
+  test: calls token endpoint with the `client_credentials` grant type.
 * `make user_info`  
   setup: creates human users and signs them in  
   test: calls user info endpoint using the given humans
@@ -43,4 +46,7 @@ Before you run the tests you need an initialized user. The tests don't implement
   test: creates a human, updates its profile, locks the user and then deletes it 
 * `make introspect`  
   setup: creates projects, one api per project, one key per api and generates the jwt from the given keys  
-  test: calls introspection endpoint using the given JWTs
+  test: calls introspection endpoint using the given JWTs
+* `make add_session`  
+  setup: creates human users  
+  test: creates new sessions with user id check