Add cache-settings to client-builder.

[Splingush]

Hello!

This adds a method to the client-builder for setting the cache-settings.
It only adds a method, but does not modify the existing cache_update_timeout.

Usage-example:

 Client::builder("token")
        .cache_settings(|cache| cache.max_messages(100))

Without these changes, the max_messages have to be set after the client has been created, via

client: Client = // [...]
client.cache_and_http.cache.set_max_messages(count).await

The settings were not modified, and currently only include the max_messages. But it might be future-proof for adding to it.