Add new empty 'arti-bin-shared' crate

This adds a new empty 'arti-bin-shared' crate. See #1750.

'arti-' crate prefix: There's a question of whether it should use the 'tor-' or 'arti-' prefix. The semver policy makes no statements about any difference between the stability of 'tor-' and 'arti-' crates. Additionally, we have both 'arti-' and 'tor-' crates that are unstable (0.x version numbers). Since this new library is meant to support common behaviour between all "arti" binaries, and presumably non-"arti" binaries will do their own thing with their own behaviour, I think it makes sense to use the 'arti-' prefix here. (Edit: I see a comment "We have decided that 'tor-*' crates have much more unstable APIs than 'arti-*' in "CONFIG-NOTES.md".)

stability: I've tried to make it clear in the README that this is meant only for internal use by arti binary crates. I think we want to be able to make breaking changes without any concern about external users.

/cc @nickm