Description:

Add a new feature: Serial to TCP bridge (similar to esp-link). This allows to remotely communicate to a MCU through ESP8266. Needs #define USE_TCP_BRIDGE

It adds 2 GPIO types: TCP Tx (208) and TCP Rx (209) and can work with hardware or software serial.

Commands:

• TCPBaudRate <x>: sets the baudrate for serial (only 8N1 mode), min 1200, max 115200 by 1200 increments.
• TCPStart <port>: listens to port <port>. This features supports 2 parallel TCP connexions, which can be useful if you need a terminal + a specific protocol (like XMODEM). The 3rd connection will disconnect an previous connection. The number of parallel connections is a compile-time option.
• TCPStart 0 or TCPStart: shuts down the TCP server and disconnects any existing connection.

For security reasons, the TCP bridge is not started at boot, and requires an explicit TCPStart command (can be automated with Rules).

Size impact:

• Flash: +1.3kb
• Ram: +64 bytes

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR.
• The code change is tested and works on core ESP8266 V.2.7.1
• The code change is tested and works on core ESP32 V.1.12.2
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass