Make publicKey optional when signTransaction

[mjuchli-da]

Make publicKey optional when signTransaction. Instead, some signing providers might want to use the name as an identifier.
Potentially, take Key as the identifier but have the publicKey as optional (or the name as optional [either|or]).

splice-wallet-kernel/api-specs/openrpc-signing-api.json

Line 28 in 71625bd

"publicKey": {