Add a script that sets up a trusted MITM proxy (#24698)

In an effort to squash bugs like:
https://github.com/zed-industries/zed/issues/19620, and improve
confidence on PRs like:
https://github.com/zed-industries/zed/pull/24656, I created this little
test script using `mitmproxy` to simulate the situation.

Unfortunately, I don't see any issues with our current usage of the
local certificate store using this script. But I'd like to have it as a
base to build off of.

Release Notes:

- N/A