Set BLE connection TX power

[edwardatki]

I recently had a board manufactured based on an NRF52840 and have experienced very poor BLE range no doubt due to a poorly designed antenna on my part, however in my search for a short term fix I discovered that there is currently no way to set the TX power for BLE connections.

It is possible to set it for advertisements with the Advertisement.tx_power property but as far as I can tell this does not carry over once a connection is established.

Given how BLE connections are implemented in circuitpython, I would suggest that connection TX power should be set globally rather than per connection. Perhaps this would best be done by finally making BLERadio.tx_power settable.

[dhalbert]

As noted above and in #5988, transmit power is set per connection. This is not exactly a dupe of #5988 because that proposes power control per connection. Adding both a default connection power and per-connection power I think makes sense.