Expose signers info for transaction/operation

[nebolsin]

In certain cases it would be handy to explore not only the source account of the transaction, but also the exact set of signers (think of a payment from multi-sig 2-of-3 acc — I want to show who exactly authorized it).

• Add signers link to the transaction (flat array of signers resolved from the signature hints attached to the transaction envelope)
• Same for operation, although in this case we need to return a subset of transaction signers who authorized this operation (basically intersect tx.signers with op.sourceAccount.signers)
• Reverse edge also seems reasonable, like query all the operations authorized (signed) by a given signer public key (or account)

This task will probably be much easier with the internal ingestor and ES / Dgraph, so lets postpone it for now.