rpc: sendtoaddress and sendmany should return PSBT for watch-only wallet

[Sjors]

Similar to bumpfee #16373, I would like sendtoaddress and sendmany to return a PSBT when used on a watch-only wallet. These methods are easier to use than walletcreatefundedpsbt.

I added this functionally as part of external signer support in #16546, which helps keep that PR short. But I think it makes sense as a standalone PR.

[Sjors]

Probably best built on top of #18115.

cc @instagibbs, @achow101: how does this fit in the (Legacy/Descriptor)ScriptPubKeyMan architecture? Right now these methods use an RPC helper SendMoney, which in turn calls wallet->CreateTransaction and pwallet->CommitTransaction(). What's the appropriate place to make it call either FillPSBT() or SignTransaction() on the ScriptPubKeyMan?

[Sjors]

For sendmany the change is pretty straight-forward, see 2a04b96

[promag]

From #16378

This paves the way for deprecating sendtoaddress and sendmany though there's no rush. The only missing feature compared to these older methods is adding labels to a destination address.

But here you want to add more features to these RPC 😅

[Sjors]

Things change quickly! Closing this.