Make it easier to generate Retry-After headers

[jvanasco]

I could be reading the docs and code wrong, but it seems the only way to generate "retry-after" headers for polling is by actually validating and using the sleep vars; i.e. this implies it is impossible to generate these headers with PEBBLE_VA_ALWAYS_VALID=1 or PEBBLE_AUTHZREUSE=100, which complicates the testing environment needed to ensure the header values are respected.

It would be nice if a PEBBLE_XXX variable could be used in conjunction with the above commands to somehow generate the retry-after headers. Perhaps a flag could just simply stall these authorizations for X seconds before transitioning to valid; or even if there is something that could give every endpoint/request an X seconds retry-after response on a first access - then a Client Author could spin up a "retry" version of pebble to ensure the throttles are being properly respected.