mcclient manifest commands

[yusefnapora]

This adds some mcclient commands for working with manifests from mediachain/concat#116

• mcclient manifest get [remotePeer] prints the array of existing manifests (optionally for the remote peer)
• mcclient manifest add [filename] reads a signed manifest from stdin or filename and adds it to the node if it doesn't exist already
• mcclient manifest remove [filename] reads the manifest and removes it from the node if it exists on the node already
• mcclient manifest self prints the "node manifest" body, which can be fed into mcid sign to produce a signed manifest.

I'd like to also add aleph support, but that can go in a new PR

[parkan]

Hmm yeah let's call manifest node something else. prepare? blank? challenge? ingot? preimage? affidavit?

p.s. this is not really the right command but I really want to use autopen for something

[vyzo]

maybe manifest self?

[vyzo]

It might also make sense to have a manifest set command to unconditionally set the manifests, without having to fight with remove arguments.