feat: support datetime_field as expr for bigquery

[chenkovsky]

bigquery supports

SELECT DATETIME_TRUNC(CURRENT_DATETIME, WEEK(MONDAY))

WEEK(MONDAY) should be parsed to datetime_field

[iffyio]

double checking the intent of the changes: my understanding is that currently given SELECT DATE_TRUNC(CURRENT_DATE, WEEK(MONDAY), WEEK(MONDAY) should be parsed as a function with name WEEK and having a single identifier MONDAY as argument? Is that indeed the case (vs e.g the parser not being able to handle such a statement)

[chenkovsky]

currently, DAY will be parsed as ident, week will be parsed as function. I think this is flawed.

1. when transpiling bigquery sql to another dialect, DATE_TRUNC(CURRENT_DATE, DAY) should be transpiled to DATE_TRUNC(CURRENT_DATE,'day'), but currently, it will be transpiled into DATE_TRUNC(CURRENT_DATE, DAY).
2. when compiling sql to logical plan, planner will try to find column called DAY, MONDAY, and function called WEEK. we should tell planner this is not function or column. I think we should convert it into str when planning.

[iffyio]

Ah I see, I think the current behavior of having it parsed as a function is preferable since the construct is syntatically a function - there's quite a lot of such special scenarios across dialects and it would likely be unwieldy to cover them consistently and I think out of scope for the parser as well

[chenkovsky]

thanks. if it's out of scope， i will close this pr. it can also be solved with a postprocessor.