[Feature request] how to join extra dbs, eg:mysql tables

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/iotdb/issues) and found nothing similar.

### Motivation

how to join extra database, eg:mysql、oracle tables

### Solution

_No response_

### Alternatives

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start by reviewing the existing database join and external-table capabilities in Apache IoTDB, then determine how MySQL and other external databases should be supported. The issue provides no target files, tests, proposed behavior, or acceptance criteria, so completion would require defining the design and expected behavior first.

Written by the indexing model from the issue text.