R1:f950985cfd7c

Allow Passphrase to generate SSH keypairs and extact public keys from private keys

Summary:
Ref T4587.

- Add an option to generate a keypair.
- Add an option to view the public keys for existing keypairs.

Test Plan:
- Generated keypairs.
- Viewed public keys.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4587

Differential Revision: https://secure.phabricator.com/D8515