feat: pass custom headers to `RESTClient` #5128

Job	Run time
run-core-unit-tests (test_client.py)	1m 3s
run-core-unit-tests (test_local_client.py)	1m 1s
run-core-unit-tests (test_client_legacy.py)	2m 29s
run-core-unit-tests (test_server.py)	1m 11s
run-core-unit-tests (test_managers.py)	53s
run-core-unit-tests (test_o1_agent.py)	1m 21s
run-core-unit-tests (test_tool_rule_solver.py)	50s
run-core-unit-tests (test_agent_tool_graph.py)	1m 0s
run-core-unit-tests (test_utils.py)	53s
run-core-unit-tests (test_tool_schema_parsing.py)	2m 5s
run-core-unit-tests (test_v1_routes.py)	47s
misc-unit-tests	3m 48s
	17m 21s

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