Add BLE GAP/GATT support to the dev-esp32 branch

[pjsg]

Fixes #3469

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [x] I have thoroughly tested my contribution.
• [x] The code changes are reflected in the documentation at docs/*.

This is the first pass at BLE support. There is some more tidyup to do, and also enabling it is slightly tricky as it is built on top of the NimBLE stack -- and I can't find a way to force enable that stack from the Kconfig.

You can startup and shutdown the BLE stack multiple times and it doesn't obviously leak memory.

It seems to work in my test environment.

Interested in comments.

[pjsg]

I'd like to get this merged -- any more input?

[marcelstoer]

As I can't assess C code I added Johny as a reviewer.

[jmattsson]

Nice, is there anything else outstanding other than sorting out the merge issues?