GoogleTestAddTests: Refactor into callable method

Move test discovery logic into new gtest_discover_tests_impl method
and make GoogleTestAddTests aware of whether it is being launched in
CMake's script mode.

When launched in script mode, gtest_discover_tests_impl is called
passing arguments obtained from the definitions passed into the call to cmake.
(i.e. cmake -P GoogleTestAddTests -D <arg1> -D <arg2> ...)

This preserves the existing behavior assumed by GoogleTest.cmake.
Unit tests are unchanged and still pass.

Looking ahead, it also allows GoogleTestAddTests to be included in generated files
and call gtest_discover_tests_impl to perform test discovery at test runtime
with the new PRE_TEST discovery mode introduced later in this branch.

My original approach attempted to call execute_process(cmake -P ...) in
the generated file, the same way POST_BUILD is doing, but I ran into
difficulties serializing the command arguments correctly.

By exposing a way to call gtest_discover_tests_impl directly from our generated file,
we remove a layer of shell quoting / parsing that our arguments have to survive,
which simplifies the act of producing a generated file that behaves the
same as its POST_BUILD counterpart.

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