Brotli compression support on client

Add Brotli Cmake support (#584)

Had to create a custom FindBrotli package, as not all users have
PkgConfig installed (which Brotli uses). This file gets installed
alongside httplibConfig.cmake for the end-users convenience.

Set BROTLI_USE_STATIC_LIBS to ON if you want to find the static libs
instead of default shared.

Adds the HTTPLIB_REQUIRE_BROTLI (default off) and HTTPLIB_USE_BROTLI_IF_AVAILABLE
(default on) options, which work in the same manner as the other optional/required
dependency options.

Moved the scattered linking and definitions to a single call.

Updated some documentation about the new options.

Improved the in-tree support by setting the HTTPLIB_IS_USING_XYZ
variables in the main CMakeLists (as well as having them in the
httplibConfig.cmake file).

Fixes #582