Add /resolve-timestamp to portal API

[kalabukdima]

See subsquid/sqd-portal#72

[mo4islona]

Path is not very canonical

GET /datasets/<dataset>/resolve-timestamp/<timestamp>

I would suggest something like

GET /datasets/<dataset>/timestamps/<timestamp>/block

Meaning: “Give me the block that corresponds to this timestamp.”

[belopash]

Nut sure if it should be 404, sometimes it's inconvinient to work with error codes. Maybe should be something like

{ "blockNumber": null }

also probably we should return hash + number instead of just number

[kalabukdima]

Nut sure if it should be 404, sometimes it's inconvinient to work with error codes. Maybe should be something like

{ "blockNumber": null }

How about just null in the response so that it's always an optional number? We already have top-level null returned from /head in case of missing data. I understand that JSON object would be more extensible, but I don't see why this endpoint may ever get more complicated than that.

also probably we should return hash + number instead of just number

Why do we need a hash?