Export chat should be limited by homeserver settings

### Your use case

The ability to [export chats](https://github.com/matrix-org/matrix-react-sdk/pull/6081) was recently introduced, which raises some interesting privacy concerns:

- Users may expect that in E2EE rooms that their conversations can not be shared easily in an unencrypted format
- Users may not expect to have their conversations saved without their explicit permission
- Homeserver may not wish to allow users to easily export E2EE chats in an unencrypted format
- Homeserver privacy polices may be incompatible with the ability to export other users data (i.e. their chats) in such a format.

#### What would you like to do?
Homeserver should be able to specify whether chats can be exported (i.e. always, unencrypted rooms only, none) for rooms in their homeserver and clients should respect them.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_