Add the string functions `TRIM` / `LPAD` / `RPAD`

### What & why / 来龙去脉

GeaFlow's DSL already ships 27 string functions (`LTrim`, `RTrim`, `Substr`,
`Concat`, …), but three everyday ones are missing: `TRIM` (strip whitespace from
both ends), and `LPAD` / `RPAD` (pad a string to a given length). These come up
constantly in data cleaning, so adding them is an immediately useful first PR.

DSL 已经有 27 个字符串函数,但还缺三个数据清洗里天天用的:`TRIM`(去首尾空格)、
`LPAD`/`RPAD`(左右填充到指定长度)。补上它们,第一个 PR 就能让人立刻用上。

### The task / 要做的事

Add three scalar functions that match standard SQL behavior:
- `trim(str)` — strip leading/trailing whitespace
- `lpad(str, len, pad)` / `rpad(str, len, pad)` — pad to a target length

### Where to look / 改哪些文件

1. Copy the shape of
`geaflow-dsl/geaflow-dsl-plan/src/main/java/org/apache/geaflow/dsl/udf/table/string/LTrim.java`.
In the same folder add `Trim.java`, `LPad.java`, `RPad.java` — extend `UDF`,
add the `@Description` annotation, implement `eval(...)`, and mind the null case.
2. Register three lines in `BuildInSqlFunctionTable.java`:
`.add(GeaFlowFunction.of(Trim.class))`.
3. Add `function_trim.sql` (etc.) under `query/` and matching `.txt` files under `expect/`.

### Done when / 完成标准

- [ ] All three functions implemented and registered, each with `@Description`
- [ ] `null` input returns `null` rather than throwing
- [ ] New `.sql` tests + `.txt` expectations pass via `mvn test` on the right test class
- [ ] Behavior matches MySQL / standard SQL — paste a quick comparison in the PR