Feature Request: Add Support for toStartOf*

[tomh4]

A really usefull aggregation method in Clickhouse is the toStartOf* logic when working with DateTimes.
Especially toStartOfInterval (which basically covers any of the others)
https://clickhouse.com/docs/reference/functions/regular-functions/date-time-functions#toStartOfInterval

It would be really handy to have this in the client as well!

Thanks for all the good work here!

[tomh4]

Are you accepting PRs?
I added this functionality and tested it, as I needed it in a specific project.
Adhered to the guidelines and style of the existing repo and added it in a similar way as e.g. the ClickHouseJsonDbFunctionsTranslator

[alex-clickhouse]

Yes, please go ahead and open a PR. I think there are other ClickHouse datetime functions that we should also add support for, but we can handle those separately.

[tomh4]

thus, with the PR #57 merged, we can close this right?