fix(expr): to_timestamp should return timestamptz

[xiangjinwu]

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What's changed and what's your intention?

Fix #7780

Backward compatibility: The old to_timestamp(varchar, varchar) -> timestamp would not be generated by frontend, but is still accepted by backend, despite lacking the constant template optimization.

Tested locally: (1) create a mview with to_timestamp on main branch; (2) kill the cluster, switch to PR branch, and start cluster; (3) query the mview, insert new rows to its upstream and query again

Given the initial request to support to_timestamp comes from superset, it is likely this expr is not used in a mview yet. If we do find it is necessary to optimize this legacy variant with constant template as well, we can refactor ExprToTimestampConstTmpl with generic.

The following styling issues will be handled in a followup PR:

• timezone vs time_zone
• to_timestamp1 -> char_to_timestamptz
• to_timestamp -> sec_to_timestamptz

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)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

• My PR contains user-facing changes.

Types of user-facing changes

• SQL commands, functions, and operators

Release note

to_timestamp(input_string, format) returns timestamptz, aligning with PostgreSQL.

[codecov]

Codecov Report

Merging #11018 (ad95100) into main (ba622f0) will increase coverage by 0.01%.
The diff coverage is 37.89%.

@@            Coverage Diff             @@
##             main   #11018      +/-   ##
==========================================
+ Coverage   69.89%   69.90%   +0.01%     
==========================================
  Files        1312     1312              
  Lines      224186   224266      +80     
==========================================
+ Hits       156700   156779      +79     
- Misses      67486    67487       +1     

Flag	Coverage Δ
rust	69.90% <37.89%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/frontend/src/expr/session_timezone.rs	66.17% <0.00%> (-4.14%)	⬇️
src/expr/src/expr/expr_to_timestamp_const_tmpl.rs	2.27% <5.12%> (+0.48%)	⬆️
src/expr/src/vector_op/to_timestamp.rs	64.86% <69.76%> (+64.86%)	⬆️
src/expr/src/expr/build.rs	80.53% <75.00%> (-0.16%)	⬇️
src/common/src/types/timestamptz.rs	74.07% <100.00%> (+3.70%)	⬆️

... and 4 files with indirect coverage changes

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[wangrunji0408]

LGTM!