[Version-unspecific] StreamingSwap endpoints' source_asset and target_asset and destination fields

[Version-unspecific]

Intended to close #1764 (closed)
'Add inbound amount/asset and destination to the streaming API response'.

Since showing source_asset, target_asset as well (as I would personally like to see both rather than only one).

As always, feedback is welcome.

For instance, this gets the source_asset from the corresponding MsgSwap,
treating the existing deposit field as the inbound amount,
but by 'inbound amount' does the Issue desire the pre-affiliate-fee inbound amount from GetObservedTxInVoter?

(My own impulse at this time is for the streaming swap endpoints to show only amounts and assets of the swap itself,
rather than amounts or assets that are not taking part in the swap.)

---

Fullnode testing with pruned snapshot height 13545069:

Swap queue context:
https://thornode-v1.ninerealms.com/thorchain/queue/swap?height=13545069

Current code (at time of writing):

https://thornode-v1.ninerealms.com/thorchain/swaps/streaming?height=13545069

https://thornode-v1.ninerealms.com/thorchain/swap/streaming/00B41741B140860AED7A85915802A2F6B940291FB821442D58A8B35E1A40A171?height=13545069

With commit bb62037a instead:

openapi/openapi_test.go

@@ -43,6 +43,7 @@ func (Test) TestJSONSpec(c *C) {
 	assertJSONStructTagsMatch(c, types.QueryQueue{}, gen.QueueResponse{})
 	assertJSONStructTagsMatch(c, types.QuerySaver{}, gen.Saver{})
 	assertJSONStructTagsMatch(c, types.MsgSwap{}, gen.MsgSwap{})
+	assertJSONStructTagsMatch(c, types.StreamingSwap{}, gen.StreamingSwap{})
 
 	// txs
 	assertJSONStructTagsMatch(c, types.TxOut{}, gen.KeysignInfo{})