Problem: web wallets do not allow signing raw messages. Instead,
they require binary payloads in a specific format.

Solution: support Micheline-style signatures, i.e. signatures
supported by wallets like Beacon.

Users can now use Micheline or raw signatures by specifying
the `signature.signingType` field to "micheline" or "raw".
By default, "raw" is assumed.

Co-authored-by: Mike Hukiewitz <70762838+MHHukiewitz@users.noreply.github.com>

Problem: if an invalid message somehow managed to reach
the pending message collection, the message would be retried
indefinitely logging exceptions on each run.

Solution: drop invalid messages.

We now provide a CLI tool that integrates all the operations
commonly performed by node operators. Currently, this CLI
allows to:

* generate private keys for the node, replacing a functionality
  that was implemented in the CCN main app directly.
* run migrations, replacing the config updater script.

Added a new command to the CCN operator CLI to run a garbage
collector on local storage. The new `gc run` command lists
all the files that are not linked to any message or permanent
pin and deletes them.

Using the --verbose option, the command will print more details
on the files it will preserve and delete. The --dry-run option
allows to run the GC without actually deleting any file.