Allow users to specify an existing web client

[qwerty01]

Currently, the crate creates its own webclient when you use one of the backend features. In order to use an existing client, users currently have to implement BeatSaverApi either for the client they're using, or for a wrapper struct. It would be more convenient to provide a trait that adds new(client: Client) for each of the backends.

[qwerty01]

This will work via a From<Client> implementation. One area that may be useful is setting user agents, but this might not be practical since some clients don't support a client-wide user agent and the user agent change is set at request time.
At the moment, only the Reqwest backend will support custom user agents via the From<Client> implementation.