JsonFileWriter fails with NPE after VectorSchemaRoot syncSchema

Hey there :)

Repro:
- Open VSR
- Do something that'll change the `Field` of one of the vectors in the VSR (promotable writer, etc)
- `vsr.syncSchema()`
- Open JsonFileWriter, `start`, `write` -> NPE

Looks like the cause is that the `fieldVectorsMap` cache isn't updated in `syncSchema`? Possibly a few other places - `addVector`/`removeVector` too?

Cheers,

James

**Reporter**: [James Henderson](https://issues.apache.org/jira/browse/ARROW-11385) / @jarohen

_{**Note**: *This issue was originally created as [ARROW-11385](https://issues.apache.org/jira/browse/ARROW-11385). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

重現報告中提到的涉及 VectorSchemaRoot、syncSchema 和 JsonFileWriter 的序列，然後檢查 fieldVectorsMap 快取以及報告中提到的 addVector/removeVector 路徑。完成的標準是：vector Field 發生變更且 syncSchema 在沒有 NPE 的情況下完成後，JsonFileWriter 可以啟動並寫入。

由索引模型根據 Issue 內容生成。