Allow transforms to set their own identifiers

[staltz]

Context

This work is motivated by PPPPP using secret-stack with new identifiers for peers based on their public key.

Problem

Currently, secret-stack hard codes IDs @<BASE64>.ed25519.

Solution

I first thought about adding a special config that allows you to customize the peer ID to be whatever you want, but this was more complex to implement and also requires you to configure such if you want a new ID system. That's not the secret-stack way, which so far has been mostly driven by plugins.

So what I went with instead is that secret-stack doesn't (or shouldn't) have a secret-stack-wide ID, but instead the transform plugins have the responsibility of determining the IDs. This is already true for secret handshake, because lib/plugins/shs is the one who sets api.id.

See the tests changed in this PR for a simple example.

So now

• shs => sets "legacy" api.id = "@<BASE64>.ed25519"
• shs => sets "modern" api.shs.pubkey = "<BASE64>" (this is the same as secret file public field)
• shse => sets "modern" api.shse.pubkey = "<BASE58>"

So instead of using ssb.id, the modern way will be ssb.shs.pubkey or with PPPPP it's ppppp.shse.pubkey.

[staltz]

[x] change those to getters, so they cannot be written to

Great idea! I did that now in a new commit.

[ ] do you want a Buffer returned instead? Or as well?

I thought about this, but Buffer has the downside of not being as easy to compare as strings are. Also, transforms (shs, shse) are free to define their own identifiers, and this means that if you want to customize the identifier then it's just a matter of making your own transform plugin (which is not that hard to write, if you look at the shs plugin in this repo).

Plugins (and swapping plugins for compatible ones) sounds like the secret-stack way, instead of forcing a standard everywhere (which is what ssb.id is).