Add today alias for current_date

[smallzhongfeng]

Which issue does this PR close?

Closes #.

Rationale for this change

ck support udf today https://clickhouse.com/docs/en/sql-reference/functions/date-time-functions#:~:text=Accepts%20zero%20arguments%20and%20returns%20the%20current%20date%20at%20one%20of%20the%20moments%20of%20query%20analysis.%20The%20same%20as%20%E2%80%98toDate(now())%E2%80%99.

duckdb https://duckdb.org/docs/archive/0.9.2/sql/functions/date#:~:text=1992%2D04%2D01-,today(),-Current%20date%20(start

What changes are included in this PR?

Add a new name for current_date

Are these changes tested?

Are there any user-facing changes?

[comphead]

Thanks @smallzhongfeng for this PR
Please add also tests and user documentation scalar_functions.md

[smallzhongfeng]

Thanks @smallzhongfeng for this PR Please add also tests and user documentation scalar_functions.md

Thanks. Updated.

[comphead]

Thanks @smallzhongfeng for this PR Please add also tests and user documentation scalar_functions.md

Thanks. Updated.

Please add today to the function list as well, see date_trunc, datetrunc as an example.

Also the tests are needed, we can likely use timestamps.slt sqllogic file as the test location

[smallzhongfeng]

I add a test, because the function today is real-time, I can only compare it with the function now.

[Dandandan]

Could you add it for today as well?

[alamb]

Thank you @smallzhongfeng

[comphead]

Thanks @smallzhongfeng
Please add today to the function list in the doc, like in https://github.com/apache/arrow-datafusion/blob/9be75f71eb59a359674669f5c2688f49f44b4050/docs/source/user-guide/sql/scalar_functions.md?plain=1#L1282

[alamb]

Thank you again @smallzhongfeng -- this looks great

[Dandandan]

Thank you @Weijun-H