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/multipart.h

@@ -0,0 +1,79 @@
+#ifndef CPR_MULTIPART_H
+#define CPR_MULTIPART_H
+
+#include <cstdint>
+#include <initializer_list>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+namespace cpr {
+
+struct File {
+    explicit File(std::string&& filepath) : filepath(std::move(filepath)) {}
+    explicit File(const std::string& filepath) : filepath(filepath) {}
+    const std::string filepath;
+};
+
+struct Buffer {
+    using data_t = const unsigned char*;
+
+    template <typename Iterator>
+    Buffer(Iterator begin, Iterator end, std::string&& filename)
+            // Ignored here since libcurl reqires a long.
+            // There is also no way around the reinterpret_cast.
+            // NOLINTNEXTLINE(google-runtime-int, cppcoreguidelines-pro-type-reinterpret-cast)
+            : data{reinterpret_cast<data_t>(&(*begin))}, datalen{static_cast<long>(
+                                                            std::distance(begin, end))},
+              filename(std::move(filename)) {
+        is_random_access_iterator(begin, end);
+        static_assert(sizeof(*begin) == 1, "only byte buffers can be used");
+    }
+
+    template <typename Iterator>
+    typename std::enable_if<std::is_same<typename std::iterator_traits<Iterator>::iterator_category,
+                                         std::random_access_iterator_tag>::value>::type
+    is_random_access_iterator(Iterator /* begin */, Iterator /* end */) {}
+
+    data_t data;
+    // Ignored here since libcurl reqires a long:
+    // NOLINTNEXTLINE(google-runtime-int)
+    long datalen;
+    const std::string filename;
+};
+
+struct Part {
+    Part(const std::string& name, const std::string& value, const std::string& content_type = {})
+            : name{name}, value{value},
+              content_type{content_type}, is_file{false}, is_buffer{false} {}
+    Part(const std::string& name, const std::int32_t& value, const std::string& content_type = {})
+            : name{name}, value{std::to_string(value)},
+              content_type{content_type}, is_file{false}, is_buffer{false} {}
+    Part(const std::string& name, const File& file, const std::string& content_type = {})
+            : name{name}, value{file.filepath},
+              content_type{content_type}, is_file{true}, is_buffer{false} {}
+    Part(const std::string& name, const Buffer& buffer, const std::string& content_type = {})
+            : name{name}, value{buffer.filename}, content_type{content_type}, data{buffer.data},
+              datalen{buffer.datalen}, is_file{false}, is_buffer{true} {}
+
+    std::string name;
+    std::string value;
+    std::string content_type;
+    Buffer::data_t data{nullptr};
+    // Ignored here since libcurl reqires a long:
+    // NOLINTNEXTLINE(google-runtime-int)
+    long datalen{0};
+    bool is_file;
+    bool is_buffer;
+};
+
+class Multipart {
+  public:
+    Multipart(const std::initializer_list<Part>& parts);
+
+    std::vector<Part> parts;
+};
+
+} // namespace cpr
+
+#endif