Add more positive & negative E2E tests for identity (multiparty mode and non-multiparty mode)

The E2E identity tests are currently a bit thin. As a very central piece of FireFly's architecture and security model, they really should have more positive tests, and particularly some negative tests to verify disallowed operations.

**Background on identities:**
* Identities in FireFly are senders and receivers of data
* They are of type "org", "node", or "custom"
* They are described with a [DID](https://www.w3.org/TR/did-core/)
* Org and custom identities are _on-chain_ identifiers with attached blockchain keys; node identities are _off-chain_ identifiers with an attached dataexchange ID
* Orgs may have children of any type, custom identities may only have custom identity children, nodes may not have children
* Each FireFly instance that participates in a multi-party system _must_ have a root "org" identity with at least one "node" identity child
* When sending an on+off-chain message (private or broadcast), you can choose your on-chain (org or custom) and off-chain (node) identity
* When receiving an on+off-chain message, the node identity _must_ be a child of the on-chain identity _or_ one of that identity's ancestors, and both pieces must be signed appropriately by those identities
* Slight aside: the "node" terminology is overloaded, as we sometimes also refer to FireFly itself as a "node" - but that is now somewhat distinct from the special "node identity" established between FireFly and dataexchange

**What we test today:**
* The multiparty on+off-chain [messaging suite](https://github.com/hyperledger/firefly/blob/main/test/e2e/multiparty/onchain_offchain.go) thoroughly uses the baseline identity functionality across two FireFly members - ie 1 root org with 1 node each
* The multiparty [identity suite](https://github.com/hyperledger/firefly/blob/main/test/e2e/multiparty/identity.go) creates "custom" child identities to the root org, and uses those child identities to send basic broadcast and private messages

**Gateway mode suite:**
We need a copy of this suite added to the "[gateway mode](https://github.com/hyperledger/firefly/tree/main/test/e2e/gateway)" (ie non-multiparty) runners. This group of tests uses a single node in isolation, which creates local definitions but does not share them with any other nodes. We need (at minimum) a basic test that creates org and child identities and then queries them back.
Note: to run the gateway suite you must run `make MULTIPARTY_ENABLED=false e2e`

**Additional tests that could be added to the existing multiparty identity suite:**
* Registering invalid identities (keys that have already been used, parent/child relationships that are not allowed, trying to register a child of someone else's org)
* Sending to invalid recipients (ie send a private message to an org+node that exist but are not actually related)
* Deeper nested identities

Negative tests should usually result in an event of type `message_rejected`. Most tests are not currently listening for this event (usually just `message_confirmed`), so the websocket listeners would have to be expanded to listen for this.