Add a signPsbt API method

[sstone]

This is intended to be used when eclair manages bitcoin core's wallet private keys. In that case, the bitcoin wallet is watch-only and can create and fund transactions or psbts but not signed them. This new API method will allow signing such transactions, which is more flexible than the existing sendonchain API call.

[t-bast]

I don't understand when this is going to be used? We don't want to sign arbitrary PSBTs outside of internal flows, do we?

[sstone]

There are specific situations where we need to build transactions manually and sendonchain would not be enough. Having eclair manage bitcoin core keys does not really make sense without some kind of secure runtime for eclair, and in this case sensitive API calls will need to be protected as well (for example we use ledger devices to sign such API calls).

[pm47]

It's useful when we manually do the walletcreatefundedpsbt/walletprocesspsbt/walletfinalizepsbt dance in some maintenance scripts.