provide private-key via filesystem

[meejah]

For third-party tools handling their own Onion service keys, it may still be nice to provide the actual private key via a filesystem path (instead of e.g. directly in the endpoint string). Although this could be done via Tor's "HiddenServiceDirectory" option, there may be reasons to not do that (e.g. such a directory cannot be on read-only media).

This feature will add an alternative to pass the private key to txtorcon but will still use ADD_ONION / ephemeral services under the hood.

[meejah]

https://github.com/meejah/txtorcon/pull/316

[meejah]

This is released