Allow definition of custom network interfaces (#921)

* Allow definition of custom network interfaces

* Create network client interface class

* Change to PIMPL approach

* Add example for custom network clients

Why:

- Show users how to use multiple network interfaces

This change addresses the need by:

- Adding an example PIO project to use Wi-Fi and GSM/LTE

* Add WebSockets prefix to normal and secure client

Why:

- Avoid name collision
- Fix broken reconnect change

This change addresses the need by:

- Adding WebSockets prefix to all custom clients
- Marking custom client as secure in clientDisconnect()
  - Remove broken fix for reconnecting

examples/esp32_pio/CustomNetworkClient/platformio.ini

@@ -0,0 +1,25 @@
+; PlatformIO Project Configuration File
+;
+;   Build options: build flags, source filter
+;   Upload options: custom upload port, speed and extra flags
+;   Library options: dependencies, extra library storages
+;   Advanced options: extra scripting
+;
+; Please visit documentation for the other options and examples
+; https://docs.platformio.org/page/projectconf.html
+
+[env:esp32dev]
+platform = espressif32@6.11.0
+board = esp32dev
+framework = arduino
+monitor_speed = 115200
+upload_speed = 921600
+build_flags = 
+    -DCORE_DEBUG_LEVEL=5
+    -D WEBSOCKETS_NETWORK_TYPE=10
+	-D TINY_GSM_MODEM_SIM7600
+lib_deps =
+    digitaldragon/SSLClient@^1.3.2
+    vshymanskyy/StreamDebugger@^1.0.1
+	vshymanskyy/TinyGSM@^0.12.0
+    symlink://../../..