rpc: check and throw specific pubkey parsing errors in HexToPubKey

In the helper `HexToPubKey`, check for three different causes of legacy
public key parsing errors (in this order):

    - pubkey is not a hex string
    - pubkey doesn't have a valid length (33 or 65 bytes) [NEW]
    - pubkey is cryptographically invalid, i.e. not on curve
      (`IsFullyValid` check)

and throw a specific error message for each one. Note that the error
code is identical for all of them (-5), so this doesn't break RPC API
compatibility.

The helper is currently used for the RPCs `createmultisig` and
`addmultisigaddress`. The length checks can be removed from the
call-sites and error message checks in the functional tests are adapted.

src/rpc/output_script.cpp

@@ -124,11 +124,7 @@ static RPCHelpMan createmultisig()
             const UniValue& keys = request.params[1].get_array();
             std::vector<CPubKey> pubkeys;
             for (unsigned int i = 0; i < keys.size(); ++i) {
-                if (IsHex(keys[i].get_str()) && (keys[i].get_str().length() == 66 || keys[i].get_str().length() == 130)) {
-                    pubkeys.push_back(HexToPubKey(keys[i].get_str()));
-                } else {
-                    throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Invalid public key: %s\n.", keys[i].get_str()));
-                }
+                pubkeys.push_back(HexToPubKey(keys[i].get_str()));
             }
 
             // Get the output type