status: Expose datetime of most recent block

[ptrus]

We currently expose latest_update (= the time the indexer most recently processed something, even if it was an old block) and latest_block (= height of the highest processed block).

Add latest_block_time , being the timestamp when latest_block was produced. Frontend will use it for liveness checks.

Testing (locally):

curl "localhost:8008/v1/"
{"latest_block":8049056,"latest_block_time":"2022-04-11T13:12:48+02:00","latest_update":"2023-06-25T09:20:13+02:00"}


curl "localhost:8008/v1/emerald/status"
{"active_nodes":0,"latest_block":1004425,"latest_block_time":"2022-04-11T17:04:34+02:00","latest_update":"2023-06-25T09:18:49+02:00"}

[aefhm]

Happy weekend LGTM, but defer to the other gents.

[pro-wh]

lg here too

[mitjat]

Thank you, LGTM also!
Bummer that we "have to" make a separate query (instead of using a JOIN) because of the consensus/runtime distinction.

[mitjat]

Since the change is pretty small and there are no reviewer suggestions, I'll go against convention and merge this PR in. This way, it can make it into the release I'm hoping to cut in the next few hours.