Config flag to disable Secure Backup upload

[jaller94]

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)
  • 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