feat: add current_database()

[yezizp2012]

I hereby agree to the terms of the Singularity Data, Inc. Contributor License Agreement.

As title, 💧.

PLEASE DO NOT LEAVE THIS EMPTY !!!

Please explain IN DETAIL what the changes are in this PR and why they are needed:

• Summarize your change (mandatory)
• How does this PR work? Need a brief introduction for the changed logic (optional)
• Describe clearly one logical change and avoid lazy messages (optional)
• Describe any limitations of the current code (optional)

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Documentation

If your pull request contains user-facing changes, please specify the types of the changes, and create a release note. Otherwise, please feel free to remove this section.

Types of user-facing changes

Please keep the types that apply to your changes, and remove those that do not apply.

• SQL commands, functions, and operators
  support current_database() function.

Release note

Please create a release note for your changes. In the release note, focus on the impact on users, and mention the environment or conditions where the impact may occur.

Refer to a related PR or issue link (optional)

Part of #2954

[neverchanje]

lgtm

[codecov]

Codecov Report

Merging #3650 (41f9266) into main (0d7d9a9) will increase coverage by 0.00%.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##             main    #3650   +/-   ##
=======================================
  Coverage   74.40%   74.40%           
=======================================
  Files         781      781           
  Lines      110786   110788    +2     
=======================================
+ Hits        82430    82433    +3     
+ Misses      28356    28355    -1     

Flag	Coverage Δ
rust	74.40% <50.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/frontend/src/binder/expr/function.rs	85.55% <50.00%> (-0.40%)	⬇️
src/frontend/src/expr/utils.rs	98.74% <0.00%> (-0.51%)	⬇️
src/meta/src/manager/id.rs	95.50% <0.00%> (ø)
src/common/src/types/ordered_float.rs	24.90% <0.00%> (+0.19%)	⬆️
src/meta/src/barrier/mod.rs	80.08% <0.00%> (+0.20%)	⬆️
src/meta/src/rpc/server.rs	80.51% <0.00%> (+0.23%)	⬆️
src/meta/src/model/barrier.rs	90.00% <0.00%> (+3.33%)	⬆️

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more