expression: support pushing down TRIM to TiKV

[crazycs520]

Enhancement

TRIM is currently not pushed down to TiKV, while related functions such as LTRIM and RTRIM can already be pushed down.

From the current code and history, TRIM appears to have been disabled as part of a broader rollback of high-risk string function pushdown due to correctness concerns. At the same time, TiKV already has the
related execution support for TRIM, so the current gap is mainly that TiDB does not allow it to be pushed down.

This issue is to track re-evaluating and restoring TRIM pushdown to TiKV.

Background

• TRIM is not pushed down to TiKV today.
• LTRIM / RTRIM are already supported for TiKV pushdown.
• Historically, TRIM was removed together with some other string functions when there were correctness concerns around string function pushdown.
• The rollback context can be found in:
  • issue the results of ord function pushed down to tikv is different with tidb #33391: the results of ord function pushed down to tikv is different with tidb #33391
  • PR expr: add function push down results comparing testcase #33401: expr: add function push down results comparing testcase #33401
• Earlier related history for TRIM pushdown can be found in:
  • PR expression/expression: add pushdown functions #26786: expression/expression: add pushdown functions #26786
• A more recent example of restoring a string function pushdown capability independently is:
  • issue Support pushdown lower/upper to TiKV #48170: Support pushdown lower/upper to TiKV #48170
  • PR expression: pushdown lower/upper to tikv #49045: expression: pushdown lower/upper to tikv #49045

What we want

Support pushing down TRIM to TiKV, with correctness as the first priority.

Scope

This issue is only for TRIM pushdown support.

It does not aim to restore all previously disabled string function pushdown in one batch.

Notes

Before enabling it again, we should make sure pushdown and non-pushdown paths return the same results for representative TRIM cases.

[panmuyun]

I will do this issue.