Add support for descriptor wallets in ricochet scripts

[kristapsk]

Ricochet scripts currently use dumpprivkey which is supported only in legacy wallets, user will have error with descriptor wallet:

error code: -4
error message:
Only legacy wallets are supported by this command

[kristapsk]

Legacy wallet removal will be one of priority projects for Bitcoin Core 28.0. https://github.com/bitcoin/bitcoin/issues/29465#issuecomment-1986888307