working BLE support

[burnsed]

First I just want to say the RTK Express is super awesome and at a price point well below the competition. The fact that it uses an ESP32 and is open source make it a dream. Thank you so much SparkFun!

This change adds Bluetooth Low Energy support as a configurable option. Given the nature of BLE, I think reducing the number of messages sent and the message rate is a good idea when enabling it. Maybe I should add that in the docs somewhere?

My testing/use case involves sending a single NMEA message at 4Hz, and reading those messages from chrome (web-ble) on Android and Chrome OS. So far it is working very well for me.

The code in src/bleSerial was copied from avinabmalla's ESP32_BleSerial (Thanks for all the heavy lifting @avinabmalla!) with a small fix to the capitalization of the import line for BLEServer.

Let me know if I should change anything. This should resolve #94

Thanks again!

[nseidle]

This looks great! A few things:

• Please point at release_candidate branch, not main
• Please rebase your branch. We've been doing a lot of work so it's out of sync.
• We're working to get BLESerial out of the src dir and into its own Arduino Library repo (soon)

[gdt]

It would be nice to rename release_candidate to dev, and then I think people would be less likely to get it wrong.