[ESQL] Support basic casting for date nanos

[not-napoleon]

Description

The fastest way to get most functions to work with date nanos at all will be to cast them to a type we already support well. We'll also need a way to cast back to date nanos. There are three functions wrapped up into this ticket, because I think we'll need all three at once to sensibly test any of them. I might be wrong about that though, in which case we could break this down further.

• TO_DATETIME should accept a date nanos and return a millisecond resolution time (NOTE: We need to have a rounding convention here)
• TO_LONG should accept a date nanos and return the same value as a long
• Add a new function TO_DATE_NANOS
  • Support long -> date nanos via direct conversion (i.e. value stays the same, type changes)
  • Support datetime -> date nanos via scaling (i.e. multiply by 1,000,000). Overflow will return null and generate a warning
  • Support string -> date nanos via parsing

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)