Add linter rule to prevent redefining the service namespace in client.tsp

[timotheeguerin]

Seems to be a pattern happening more and more where things are hacked in client.tsp and the service namespace has new models, operations,etc injected into it. This completly breaks the overall spec POV

[markcowl]

• new types in a different namespace than the service namespace
• wire-changing customizations (e.g. action segment changing from operation name change)