Wallet: Implement option to sign transactions with Schnorr instead of ECDSA
We already have the cryptographic functionality to create Schnorr signatures in libsecp256k1 (see !963 (merged)), but we don't offer an option to use it in practice yet.
This issue is about making signing with Schnorr available in the wallet user interface and various RPC functions, as an alternative to signing with ECDSA. Fee calculation logic should be adjusted to account for the smaller size of Schnorr signatures. Special attention may be needed for Partially-Signed Bitcoin Transactions (PSBTs).
This issue is about adding the option. Please read #227 before enabling the option by default.
Upstream: ABC task T530