Support general GET interface

[bretg]

The audio committee has asked for an improved/generalized GET interface to Prebid Server.

Here are the requirements they've put together: https://docs.google.com/document/d/1KnXuLUcTr9NpZCGXYzOFV5B6lSj4cvd7iY1JI8FRUOw/edit

The CTV committee has also provided requirements - mostly overlapping: https://docs.google.com/document/d/1i71a_i7SZOAcyBijUiYOYCwnMpnxG_6XCrNoy9LoRqI/edit

There are 3 core components to support these requirements. This issue tracks the development of the GET interface.

[Net-burst]

Document with the proposed supported query string parameters: https://docs.google.com/spreadsheets/d/1dbSaK4zxgvuJp1bw95dqKRFkYGbJ4oqVrzmtodYrplY/edit

[bretg]

Draft tech spec outlining how the Prebid Server committee plans to respond to the requests from the Audio and CTV committees. https://docs.google.com/document/d/1Fkwym4EREfOdaTD8SHUODKwlj-vWpZWQa0eGUToyzPU/edit