R1:51153a580cd6

Implement "git-lfs-authenticate" over SSH

Summary:
Ref T7789. This implements a (probably) usable "git-lfs-authenticate" on top of the new temporary token infrastructure.

This won't actually do anything yet, since nothing reads the tokens.

Test Plan:
```
$ ./bin/ssh-exec --phabricator-ssh-user admin --ssh-command 'git-lfs-authenticate'
phabricator-ssh-exec: Expected `git-lfs-authenticate <path> <operation>`, but received too few arguments.
```

```
$ ./bin/ssh-exec --phabricator-ssh-user admin --ssh…