pull: Add support for sign-verify=<list>

pull: Add support for sign-verify=<list>

The goal here is to move the code towards a model
where the *client* can explicitly specify which signature types
are acceptable.

We retain support for `sign-verify=true` for backwards compatibility.
But in that configuration, a missing public key is just "no signatures found".

With `sign-verify=ed25519` and no key configured, we can
explicitly say `No keys found for required signapi type ed25519`
which is much, much clearer.

Implementation side, rather than maintaining `gboolean sign_verify` *and*
`GPtrArray sign_verifiers`, just have the array.  If it's `NULL` that means
not to verify.

Note that currently, an explicit list is an OR of signatures, not AND.
In practice...I think most people are going to be using a single entry
anyways.