Attach `signedBy` to the `transaction` and lookup during `execute`

[mjuchli-da]

Why is the wallet gateway actually taking the signedBy field from the executeParams and not the transaction itself here:

wallet/wallet-gateway/remote/src/ledger/transaction-service.ts

Line 544 in b7e944b

const { partyId, signature, signedBy } = executeParams

?

• signedBy should be described better as it represents the field needed in the ledger api:
  The fingerprint/id of the keypair used to create this signature and needed to verify.
• So this should always be the fingerprint used of the party during the sign step
• Currently we return the signedBy (and partyId for that matter) from the sign endpoint and use it as params during the execute endpoint.

Instead, we should attach these properties to the transaction and just look up signedBy (and possibly signedByParty)  during execute.

Please question this solution/assumption, it might be wrong