Add a new method `read_chan_signer` to `KeysInterface`

Add a new method `read_chan_signer` to `KeysInterface`

This adds a new method to the general cross-channel `KeysInterface`
which requires it to handle the deserialization of per-channel
signer objects. This allows the deserialization of per-channel
signers to have more context available, which, in the case of the
C bindings, includes the actual KeysInterface information itself.