include parties' display names for signatories, observers when present

Where parties occur in the domain model (that is, not in LF values such as contract payloads), we can supply their display names alongside, like

{identifier: string,
 displayName?: string}

One presumes that the observers and signatories of a contract ought to have access to the display names of the others.

Example event:

{
    "created": {
        "observers": [],
        "agreementText": "",
        "payload": {
            "observers": [],
            "issuer": "Alice",
            "amount": "999.99",
            "currency": "USD",
            "owner": "Alice"
        },
        "signatories": [
            {
                "identifier": "Alice",
                "displayName": "Alice Baker"
            }
        ],
        "contractId": "#1:0",
        "templateId": "b70bbfbc77a4790f66d4840cb19f657dd20848f5e2f64e39ad404a6cbd98cf75:Iou:Iou"
    }
}

[hurryabit]

Another way to reach consistency would be to leave the signatories and observers fields as is and add an additional field parties to the response. This field will contain a lookup table from party identifiers to display names. For the time being, we only fill this table with the display names of signatories and observers. For example,

    {
        "status": 200,
        "result": {
            "observers": [
                "Charlie"
            ],
            "payload": {
                "observers": ["Charlie"],
                "issuer": "Alice",
                "amount": "999.99",
                "currency": "USD",
                "owner": "Alice"
            },
            "signatories": [
                "Alice",
                "Bob" 
            ],
            "parties": {
                "Alice": "Alice from Wonderland",
                "Bob": "Bob the Builder",
                "Charlie": "Charlie Chaplin"
            }
            ...
        }
    }

I see multiple advantages with this approach:

1. It’s backward compatible and doesn’t break any existing code.
2. It avoids sending display names twice if a party is both a signatory and an observer. If we ever decide to deliver display names for all parties mentioned in the payload, this benefit would become even bigger.

What do you think?

[leo-da]

@leo-da Sounds good to me, though I emphasize this line from @hurryabit

we are not changing the active contract response. as agreed with @hurryabit we are extending /v1/parties endpoint to support lookup by by ID, the enrichment will be done on the client side.