Implement cargo espidf install

[ivmarkov]

For cases where users would like to have all of ESP-IDF (the repo itself, GCC toolchain, pip deps) pre-installed - rather than downloaded on-demand during cargo build, it might be useful to have such a command.

The existing alternative is for users to do

git clone --recursive https://github.com/espressif/esp-idf
cd esp-idf
./install.sh esp32

(... and also a few pre-requisites that we don't automate either, as per here).

Not super-difficult, but if we would like to isolate the C world even for offline, such a cargo sub-command might be useful.

Possible syntax:

cargo espidf install [--idf-ver <branch-or-tag>] [--tooling-mcu <esp32|esp32c3|...|all>]

The above would just do what we do at build time w.r.t. downloading and installing ESP-IDF.
Given that most of the ESP-IDF download & installation code is in embuild (where cargo espidf itself resides) I don't think that would be such a big lift and shift.

Eventually, we might have an update variant that updates an already existing installation. OR maybe install can do that automatically, if it detects an existing installation.