Major plugin refactor and cleanup.

Switched to POCO library for unified platform/library interface.
Deprecated the external module API. It was creating more problems than solving.
Removed most built-in libraries in favor of system libraries for easier maintenance.
Cleaned and secured code with help from static analyzers.

vendor/CPR/cpr/auth.cpp

@@ -0,0 +1,7 @@
+#include "cpr/auth.h"
+
+namespace cpr {
+const char* Authentication::GetAuthString() const noexcept {
+    return auth_string_.c_str();
+}
+} // namespace cpr