This patch implements a new mode of network tethering for Playwright
server & its clients.
With this patch:
- playwright server could be launched with the
`--browser-proxy-mode=tether` flag to engage in the new mode
- a new type of client, "Network Tethering Client" can connect to the
server to provide network traffic to the browsers
- all clients that connect to the server with the `x-playwright-proxy:
*` header will get traffic from the "Network Tethering Client"
This patch also adds an environment variable
`PW_OWNED_BY_TETHER_CLIENT`. With this env, playwright server will
auto-close when the network tethering client disconnects. It will also
auto-close if the network client does not connect to the server in the
first 10 seconds of the server existence. This way we can ensure that
`npx playwright docker start` blocks terminal & controls the lifetime of
the started container.
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