ipc: add external signer interface

[Sjors]

Allow external signer software to connect to the node. Once it registers with Init.registerExternalSigner, the node will use it.

struct SignerInfo $Proxy.wrap("interfaces::ExternalSignerInfo") {
    fingerprint @0 :Text;
    name @1 :Text;
}

# External signing service (e.g. HWI)
interface ExternalSignerService $Proxy.wrap("interfaces::ExternalSignerService") {
    enumerate @0 (chain :Text) -> (result :List(SignerInfo));
    getDescriptors @1 (fingerprint :Text, chain :Text, account :Int32) -> (receive :List(Text), internal :List(Text), error :Text, result :Bool);
    displayAddress @2 (fingerprint :Text, chain :Text, descriptor :Text) -> (address :Text, error :Text, result :Bool);
    signTransaction @3 (fingerprint :Text, chain :Text, psbt :Data) -> (signedPsbt :Data, error :Text, result :Bool);
}

Based on:

• wallet: harden external signer psbt processing, revamp mock bitcoin/bitcoin#36114 (for the new mock)