Consider merging UNIX functionality from rustls-native-certs

This proposal came up at the beginning of the year on the community Discord: Should we soft-deprecate rustls-native-certs in favor of this crate since it provides a superset of functionality? I'm moving this out of the Discord thread so its more visible and easier to track.

There are a few possible approaches here, such as

• rustls-native-certs continues to exist, as is, with a soft-deprecation and this crate will depend on it when targeting a UNIX target.
• rustls-native-certs is archived and its UNIX-specific functionality moves over here.

In the thread, @cpu mentioned they thought a README notice indicating the soft deprecation was uncontroversial. @djc brought up the first approach.

The first path would argue that we would need to maintain the crate either way, so it doesn't gain us much to merge it in here. However, the second approach would give us a lot more flexibility to implement #60 since rustls-platform-verifier could control where it reloads files from more precisely instead of the blunt API. It would also help move people towards doing the more correct thing on all OSes except Linux/BSD.

Start by reviewing the linked Discord discussion, the two proposed approaches, and issue #60, then inspect the README and the relationship with rustls-native-certs. Done requires a maintainer decision on whether to soft-deprecate, archive, or merge UNIX-specific functionality, followed by the corresponding documented and implemented migration path.