Config flag to disable Secure Backup upload

### Your use case

#### What would you like to do?
- Not uploading keys to Secure Backup.
- Not having to provide a password when restoring Secure Backup.


#### Why would you like to do it?
- Because I know another Matrix client receives and uploads all keys.
- Appservices can get an access token for an account without having to register a password for it. (merged [MSC2778](https://github.com/matrix-org/matrix-doc/pull/2778))
- If a human wants to log in to such an account, the appservice can provide an access token but no password.
- Uploading keys to Secure Backup isn't important, if the account is commonly managed and backed up by a bot.

#### How would you like to achieve it?
- A config flag `disableSecureStorageUpload` (boolean, defaults to `false`).
- You can still restore Secure Backup but it will be read-only.

### Have you considered any alternatives?

A config flag to disable all flows which require an account password and use the access token where possible or disable features. (e.g. changing the password)

### Additional context

_No response_