Adding CMake module to enable/disable transport adapters
TRANSPORT ADAPTER BUILD
Default: If no option is explicitly added, curl will be used for POSIX and WIN HTTP for WIndows
Windows: Both CURL and WIN_HTTP can be build to be used.
POSIX: Only CURL is acceptable. If WIN_HTTP is set, generate step will fail for user
Defines `BUILD_WIN_HTTP_TRANSPORT_ADAPTER` and `BUILD_CURL_HTTP_TRANSPORT_ADAPTER` for source code
Fixes#350
* Expose the ContextValueType so callers can use it to get at key values.
* Add a const char * overload for construction
* Remove explicit conversion which adds an extra move construction.
const char* implicitly converts to std::string
