[function] toStartOfInterval

[BohuTANG]

Summary

toStartOfInterval(time_or_data, INTERVAL x unit [, time_zone]) 
This is a generalization of other functions named toStartOf*. For example,
toStartOfInterval(t, INTERVAL 1 year) returns the same as toStartOfYear(t),
toStartOfInterval(t, INTERVAL 1 month) returns the same as toStartOfMonth(t),
toStartOfInterval(t, INTERVAL 1 day) returns the same as toStartOfDay(t),
toStartOfInterval(t, INTERVAL 15 minute) returns the same as toStartOfFifteenMinutes(t) etc.

Reference:
https://clickhouse.tech/docs/en/sql-reference/functions/date-time-functions/#tostartofintervaltime-or-data-interval-x-unit-time-zone

[BohuTANG]

Move to #853