[TASK] Add IDs for keys in the keystore

[Serial-ATA]

There's no (easy) way to get a key without knowing its public key. We should support allowing users to specify a key ID (such as "tangle-default").

Unresolved Questions

• How does this work with the current API?

Right now, load()ing a key expects <T as KeyType>::Public. How do we allow for loading by ID, and how do we want to handle the case of generating a key with an ID and then attempting to load it by its public key?

• How should this be handled in the filesystem storage?

Right now, the filesystem storage will name files with a hash of their public key. Should they now be named by their IDs? This ties into the previous question, how will we find those key files now if they are named by their ID and we attempt to lookup by public key?