Allow for modification of the reqwest client

[jrconlin]

Recently, we encountered an error we traced back to a lack of timeout when the reqwest object makes a call. It would be helpful to either be able to specify the timeout value, or ideally, have access to the client attribute so that reqwest behaviors can be altered when needed.

(e.g. Client::with_client(req: reqwest::Client), which is similar to Client::with_cache(...))

Alternates implementations always welcome.