[py] Set user_agent and extra_headers via ClientConfig #9920

Job	Run time
Check	9m 45s
Java	0s
Ruby	0s
.NET	0s
Rust	0s
Python / Build / Build	2m 37s
Python / Mypy	45s
Python / Lint	21s
Python / Documentation	44s
Python / Browser Tests (safari, macos) / Integration Tests (safari, macos)	23m 3s
Python / Browser Tests (chrome, macos) / Integration Tests (chrome, macos)	32m 55s
Python / Remote Tests (firefox) / Integration Tests (remote, firefox)	10m 49s
	1h 20m 59s

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