Start a protocol name registry with our custom ssh algorithm names

Start a protocol name registry in the torspec repo and document the usage and purpose of the following "protocol" names:

  +----------------------------------+--------------------+---------------------+------------------------+
  | Public Key Algorithm Name        | Public Key Format  | Private Key Format  | Purpose                |
  |----------------------------------|--------------------|---------------------|------------------------|
  | x25519@torproject.org            | [TODO link to spec | [TODO link to spec  | Arti keystore storage  |
  |                                  | describing the key | describing the key  | format                 |
  |                                  | format]            | format]             |                        |
  |----------------------------------|--------------------|---------------------|------------------------|
  | ed25519-expanded@torproject.org  | [TODO link to spec | [TODO link to spec  | Arti keystore storage  |
  |                                  | describing the key | describing the key  | format                 |
  |                                  | format]            | format]             |                        |
  |                                  |                    |                     |                        |
  +----------------------------------+--------------------+---------------------+------------------------+