Support for host RPA generation on the host

Somewhat a follow-up to https://github.com/apache/mynewt-nimble/issues/887

We're using NimBLE on several different HCI controllers, but one of the major ones doesn't support LE Privacy 1.2, which means that it relies on the host to implement RPA generation and all the IRK resolving functionality. See https://community.cypress.com/message/268835

I notice of course that the NimBLE HCI Controller does support this, so perhaps (understandably!) this is a low priority for you :) We're currently working on improving the nRF MicroPython port, and using the full NimBLE Controller+Host on that port is part of the plan.

However, before I start investigating implementing this myself (or @andrewleech), would you be interested in accepting a PR to support this in NimBLE Host? I see that Espressif have added this in their fork of NimBLE (https://github.com/espressif/esp-nimble/commit/fdfa72957d5e26ede402e96f0e1e6b608bcd9986) and it appears that Zephyr supports it too.

Start by reading the linked issue 887 and comparing the Espressif commit and Zephyr support referenced here. Done means NimBLE Host supports host-side RPA generation and IRK resolving for HCI controllers without LE Privacy 1.2 support.

Written by the indexing model from the issue text.