Support for user provided stack trace implementations

http://google.github.io/googletest/gmock_cook_book.html has examples that show stacktraces for both EXPECT_CALL invocations and the actually mocked function calls.
https://github.com/google/googletest/blob/master/googletest/include/gtest/internal/custom/README.md claims that the user may provide a stacktrace implementation by defining GTEST_OS_STACK_TRACE_GETTER_ to a (user provided) class implementing the OsStackTraceGetterInterface.
The OsStackTraceGetterInterface is not exported by a public header file in some include directory of gtest or gmock.
So it is unclear how gtest can support a user defined stack trace implementation without recompiling gtest with additional user provided source files.

**What operating system and version are you using?**
Windows 10

**What compiler and version are you using?**
Visual Studio 2019

This is actually the issue https://github.com/google/googletest/issues/1020.

Start with googletest/include/gtest/internal/custom/README.md and trace the declaration and use of OsStackTraceGetterInterface in the repository. Check how custom stack-trace implementations are expected to be supplied for Windows and Visual Studio 2019. Done means the documented user-provided implementation can be used through an available public interface without adding source files to a rebuilt gtest.

Written by the indexing model from the issue text.