ESP-IDF support

[cnf]

Adding ESP-IDF component manager support.

With this, you can add the following to your idf_component.yml, and the manager will download, compile and make available pubsub-c:

dependencies:
  pubsub-c:
    git: https://github.com/jaracil/pubsub-c
    version: "*"
    path: ports/esp-idf

the component manager doesn't like the plain cmake version in ports/esp32, hence this PR

[cnf]

Don't accept / merge yet, I walked away with it working, and now it's not building, I guess I changed somethings but I can't find it right now...