feat: add system function user, current_user and current_role

[yezizp2012]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

#10134 , this PR implements system function user. Another bug found in #10367 .

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)

Documentation

• My PR contains user-facing changes.

Click here for Documentation

Types of user-facing changes

Please keep the types that apply to your changes, and remove the others.

• SQL commands, functions, and operators

Release note

Implements system function user, current_user and current_role.

dev=> select user;
 user
------
 root
(1 row)

[codecov]

Codecov Report

Merging #10366 (e3cbe0c) into main (558cef5) will increase coverage by 0.00%.
The diff coverage is 90.32%.

@@           Coverage Diff           @@
##             main   #10366   +/-   ##
=======================================
  Coverage   70.52%   70.52%           
=======================================
  Files        1255     1255           
  Lines      213963   213988   +25     
=======================================
+ Hits       150891   150913   +22     
- Misses      63072    63075    +3     

Flag	Coverage Δ
rust	70.52% <90.32%> (+<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	87.55% <90.00%> (+0.01%)	⬆️
src/frontend/src/binder/expr/mod.rs	78.33% <100.00%> (ø)

... and 2 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more