Allow setting headers when connecting from agentk to kas

What does this MR do?

Adds the --kas-header command line option to agentk, to allow setting custom headers when connecting from agentk to KAS.

Screenshots

Command:

support/agentk token-here --kas-header="X-Header-1: value1" --kas-header="X-Header-2: value2" --kas-header="X-Header-2: value3"

Result (websocket):

Result (gRPC):

Closes #311 (closed)