Add peer storage backup

[thomash-acinq]

We use peer storage (lightning/bolts#1110) for channel backups.
We also keep the legacy channel data backup for some transition period.
Requires ACINQ/eclair#2888 on eclair's side.

[t-bast]

A note regarding backwards-compatibility: I think we don't need any kind of backwards-compat in lightning-kmp, we can do everything on the eclair side (where we need backwards-compat anyway because we need to keep working with wallets that haven't updated.

The way I think it should work is:

• in lightning-kmp, you can completely remove the old channelData TLV and only use the new backup messages
• on the eclair side, we can simply:
  • always store backup data, regardless of where it comes from (the channelData TLV or the new backup messages)
  • whenever we connect to a mobile wallet:
    • if they have the feature bit for the new backup protocol, we send them their backup using the new messages
    • otherwise, we include it in the channelData field of our channel_reestablish

Does that make sense? It seems like it should be quite simple to provide a smooth transition that way (and eventually phase out the channelData TLV field on the eclair side after 6 months or 1 year).