Allow to use local GSL

[owent]

Is your feature request related to a problem?
I found ms-gsl is a excellent tool to write modern C++ codes, and I want to use it in my project.At the same time I think it's better to let opentelemetry use the same ms-gsl.

Describe the solution you'd like
find_package(Microsoft.GSL QUIET) before set(GSL_DIR third_party/ms-gsl) and include_directories(${GSL_DIR}/include)

Describe alternatives you've considered
I find another library - gsl-lite and it seems have better compatibility for old compilers. Maybe we can use it for old compilers?

Additional context
None

[lalitb]

@owent - Can we close this issue, if #915 fixes it.

[owent]

Yes