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/include/cpr/ntlm.h

@@ -0,0 +1,15 @@
+#ifndef CPR_NTLM_H
+#define CPR_NTLM_H
+
+#include "cpr/auth.h"
+
+namespace cpr {
+class NTLM : public Authentication {
+  public:
+    NTLM(const std::string& username, const std::string& password)
+            : Authentication{username, password} {}
+};
+
+} // namespace cpr
+
+#endif