FFI: BackupDownloadStrategy cannot be set in ClientBuilder

[kegsay]

The client_builder used to set the BackupDownloadStrategy to AfterDecryptionFailure which was a sensible default. In the refactoring in https://github.com/matrix-org/matrix-rust-sdk/pull/3115 this changed, and the default now fell to the value in the enum which is Manual. I don't think this is a sensible default.

This is made worse by the fact that it's impossible to change this value now, because FFI bindings aren't created for pub(crate) fn with_encryption_settings(...) due to visibility rules.

The net result is that key backup downloading is broken on latest main EDIT: if you use the authentication service then the right defaults are set. This only applies when you use client_builder directly.

[bnjbvr]

Reopening: #3131 was a quick workaround, but it's still not possible to set it.