[improvement](fe) Add current_database builtin alias

[tonywasson]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: Add current_database() as a small compatibility alias for the existing database() builtin so portable SQL can resolve the current database name without rewrites.

Release note

Support current_database() as an alias of database().

Check List (For Author)

• Test: FE checkstyle and regression test update
  • Manual test / No need to test (with reason)
• Behavior changed: Yes (adds a compatibility alias for current database lookup)
• Does this need documentation: No

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?