Add WithContext methods to Fluent struct

[arnogeurts]

Currently the only way to have control over the maximum duration of the Post and related methods is by configuring the Timeout, WriteTimeout, RetryWait, MaxRetry and MaxRetryWait, which all together define an upper bound to the total duration. However, there is no way to define an overall timeout to the action.

A lot of networking libraries use the context deadline as an overall timeout for the operations, so it would be nice if that is also added to this library.