Allow users to enable profiling via a setting

Prior to this commit, there is no way for an end user to enable
the new profiling features other than by patching their source
code or passing an extra query parameter to a web request,
which requires them to know the precise URL / query params / POST
body of the request that they want to profile, as well as passing
all of the necessary SSL-related flags to `curl` (et al) so that
their request can be authenticated.

This commit simply adds a boolean setting called 'enable_profiling'
which can be used to turn on this functionality for all requests.

We should provide more precise ways for users to control this, but
this at least gives them a simple option until we have something
better.