Integration test for qldbsession request signing (#350)

* fix: typo in qldbsession example

* feat: test harness for QLDB signv4 service name

* fix: typo in qldbsession example

* feat: test harness for QLDB signv4 service name

* feat: integration test for qldbsession signing

* fix: request protocol version

* fix: test request protocol version

* fix: overwrite the user agent for test

Co-authored-by: Shing Lyu <shinglyu@amazon.nl>