expression: support pushing down TRIM to TiKV

## 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 #33391: https://github.com/pingcap/tidb/issues/33391
- PR #33401: https://github.com/pingcap/tidb/pull/33401
- Earlier related history for `TRIM` pushdown can be found in:
- PR #26786: https://github.com/pingcap/tidb/pull/26786
- A more recent example of restoring a string function pushdown capability independently is:
- issue #48170: https://github.com/pingcap/tidb/issues/48170
- PR #49045: https://github.com/pingcap/tidb/pull/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.

Start by reading issue #33391, PR #33401, PR #26786, and the independent restoration example in issue #48170 and PR #49045 to understand the pushdown history and correctness concerns. Compare TiDB's pushdown and non-pushdown results for representative TRIM cases, and consider the work complete when TRIM is safely pushed to TiKV with matching results.

Written by the indexing model from the issue text.