Support Map type in KV table

### Search before asking

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

### Motivation

Add Map type support for KV table scenarios to enable primary key tables with map columns.

### Solution

- Extend KV table storage format to support Map type
- Implement Map type serialization/deserialization for KV tables
- Add support for Map type in KV table read/write operations

### Anything else?

_No response_

### Willingness to contribute

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

Start by locating the KV table storage format and its read/write serialization paths, since the issue names these areas but no files or tests. Trace how existing types are serialized and deserialized. Done means KV tables support Map columns, including primary-key tables, across storage, serialization, and read/write operations.

Written by the indexing model from the issue text.