Add command to reconfigure Git to fetch private repos

[mokagio]

Currently Pocket Casts iOS and WordPress/Jetpack iOS access private Swift Packages.

The logic in WordPress is a tidied up version of what is in Pocket Casts.

Assuming the approach is sound, we might want to move the implementation here in the CI plugin.

[mokagio]

Notice that

• Pocket Casts no longer fetches private Swift packages: Remove private key swap for private repos — No longer needed pocket-casts-ios#1919
• WordPress iOS only needs access to a private package for limited time. WordPress-rs will soon(ish) become public: Remove redundant add_host_to_ssh_known_hosts command wordpress-mobile/WordPress-iOS#23427

As such, the value of this suggestion decreases dramatically.

[mokagio]

Since posting, the implementation has evolved. See https://github.com/bloom/DayOne-Apple/pull/25138.