feat: remove interval type and give datetime an alias

[fengjiachun]

1. Remove all Interval data types; for details, see Deprecate DateTime data types greptimedb#5489.
2. Make DateTime an alias of timestamp(6); for details, see fix: flush table panic when table has interval column close #3235 greptimedb#5422.
3. Minor user experience improvement: allow direct use of the Instant type to write into the Timestamp column.