Make Requests filterable

[adamziel]

Introduces a new filter, wp_http_client_request_before_enqueue, to allow modification or rejection of requests before they are enqueued.

The filter may:

• Change the request, in which case the updated request will be enqueued
• Return false to reject the request – the original request will bee enqueued and immediately rejected.

Remaining work

Only allow updating the original Request object in the filter. Reject filters that return a new Request instance – the object identity matters as Client consumers may use it to track the data flow.