Expose new shared `payjoin::directory` types

[DanGould]

Some types are shared between payjoin and payjoin-directory:

• the current payjoin_directory::db::ShortId is also used in Improve QR code encoding efficiency #389 for subdirectory IDs
• Message length constants in Pad ohttp req/res messages to consistent 8192 bytes #395 can be re-used across these

To re-use code and avoid

payjoin-directory crate would depend on payjoin crate with ONLY the payjoin/directory feature and no others. So it would only have access to ShortId and padding values.

That way payjoin defines all of the actual protocol bounds and payjoin-cli, payjoin-directory etc. are concrete implementations of that protocol, and we don't need to manage a separate release file

[nothingmuch]

After this is implemented, payjoin-directory will depend on payjoin which will depend on bitcoin, which should address this comment more meaningfully: #417 (comment)