Add FLEX Protocol Support

[Theldus]

This PR adds support for the FLEX protocol for pagers, using the public-domain, single-header library 'tinyflex'.

The tinyflex library currently supports:

• 2-FSK modulation at 1600bps
• Short and long addresses
• Alphanumeric messages up to 248 characters
• Maildrop messages

The FLEX protocol is a streaming protocol, meaning it requires a continuous data stream to maintain synchronization with the pager. However, it is possible to force re-synchronization by sending a preliminary ERS (Emergency Re-synchronization). This allows messages to be delivered even if the pager is not actively synchronized with a transmitter.

This enables behavior similar to the already-supported POCSAG protocol in rpitx.

Successfully tested on: Raspberry Pi 1B+ (Raspbian 10) + Motorola Advisor Elite.

This should address issue #277