Cleanup internal includes by splitting out some event structs

* Split out BenchmarkInfo and BenchmarkStats to their own header
* Outline BenchmarkStats<> declaration to separate header
* Split out TestRunInfo into its own header

These changes let us remove the large `interfaces_reporter.hpp`
include from `benchmark.hpp`, and replace it with
`interfaces_capture.hpp` in `run_context.hpp`.

I also cleaned out `interfaces_repoter.hpp` from reporter headers
that depend on `reporter_common_base.hpp`. This will not change
anything in the actual inclusion set, but makes it logically
more consistent.

src/catch2/internal/catch_run_context.cpp

@@ -8,8 +8,9 @@
 #include <catch2/internal/catch_run_context.hpp>
 
 #include <catch2/catch_user_config.hpp>
-#include <catch2/interfaces/catch_interfaces_generatortracker.hpp>
 #include <catch2/interfaces/catch_interfaces_config.hpp>
+#include <catch2/interfaces/catch_interfaces_generatortracker.hpp>
+#include <catch2/interfaces/catch_interfaces_reporter.hpp>
 #include <catch2/internal/catch_compiler_capabilities.hpp>
 #include <catch2/internal/catch_context.hpp>
 #include <catch2/internal/catch_enforce.hpp>