Add WebServer, WebServerSecure, HTTPUpdateServer, HTTPUpdateServerSecure (#791)

* Add HTTP-parser lib to support ESP32 WebServer
* Add WebServer from ESP32.  Only supports HTTP
* Separate HTTP server from the network server
Instead of managing the WiFiServer/WiFiServerSecure in the same object
as the HTTP handling, split them into separate objects.  This lets
HTTP and HTTPS servers work without templates or duplicating code.
The HTTP block just gets a `WiFiClient*` and works with that to only
do HTTP processing, while the upper object handles the appropriate
server and client types.
* Add HTTPS server
* Clean up some THandlerFunction refs
* Refactor into a template-ized WebServer/WebServerSecure
* Add DNSServer examples which need WebServer
* Fix CoreMutex infinite recursion crash
Core could crash while Serial debugging was going on and prints were
happening from LWIP/IRQ land and the main app.
* Add HTTPUpdateServer(Secure)
* Add MIME include, optimize WebServer::send(size,len)
When send()ing a large buffer, the WebServer::send() call would
actually convert that buffer into a String (i.e. duplicate it, and
potential issues with embedded \0s in binary data).
Make a simple override to send(size, len) to allow writing from the
source buffer instead.
* Fix WiFiClient::send(Stream), add FSBrowser example

libraries/HTTPUpdateServer/src/HTTPUpdateServer.h

@@ -0,0 +1,44 @@
+#pragma once
+
+#include <WebServer.h>
+
+template <typename ServerType, int ServerPort>
+class HTTPUpdateServerTemplate {
+public:
+    HTTPUpdateServerTemplate(bool serial_debug = false);
+
+    void setup(WebServerTemplate<ServerType, ServerPort> *server) {
+        setup(server, "", "");
+    }
+
+    void setup(WebServerTemplate<ServerType, ServerPort> *server, const String& path) {
+        setup(server, path, "", "");
+    }
+
+    void setup(WebServerTemplate<ServerType, ServerPort> *server, const String& username, const String& password) {
+        setup(server, "/update", username, password);
+    }
+
+    void setup(WebServerTemplate<ServerType, ServerPort> *server, const String& path, const String& username, const String& password);
+
+    void updateCredentials(const String& username, const String& password) {
+        _username = username;
+        _password = password;
+    }
+
+protected:
+    void _setUpdaterError();
+
+private:
+    bool _serial_output;
+    WebServerTemplate<ServerType, ServerPort> *_server;
+    String _username;
+    String _password;
+    bool _authenticated;
+    String _updaterError;
+};
+
+#include "HTTPUpdateServer-impl.h"
+
+using HTTPUpdateServer = HTTPUpdateServerTemplate<WiFiServer, 80>;
+using HTTPUpdateServerSecure = HTTPUpdateServerTemplate<WiFiServerSecure, 443>;