feat: support ANSI CURRENT_DATE/CURRENT_TIME keywords

[BohuTANG]

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Summary

• Accept ANSI realtime keywords so downstream tools can issue standard SQL without rewriting. Example (default fractional precision vs explicit precision):

  SELECT CURRENT_DATE(), CURRENT_TIME(), CURRENT_TIME(3);
  -- CURRENT_DATE()  -> 2024-04-18
  -- CURRENT_TIME()  -> 07:47:37.084123000
  -- CURRENT_TIME(3) -> 07:47:37.084

• Reuse today() / now() internals and surface the new entry points in the function registry metadata and golden listings.
• Extend sqllogic and golden coverage so we assert formatting, fractional precision handling, and error reporting (e.g. precision outside the range [0,9]).

Tests

• Unit Test
• Logic Test
• Golden Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[sundy-li]

https://neon.com/postgresql/postgresql-date-functions/postgresql-current_time

current_time should return timestamp with tz.

[BohuTANG]

https://neon.com/postgresql/postgresql-date-functions/postgresql-current_time

current_time should return timestamp with tz.

current_time now align with postgresql and snowflake.