Add ci pipeline to PR: Compile under esp-idf new build sytem

[andreaskuster]

I extended your PR #1562 with the continuous integration pipeline functionality.

There are a few other PRs open to make this working (namely, the Ethernet library has to be ported as well):

Add esp-idf cmake build system component registration arduino-libraries/Ethernet#164
After this has been resolved, we can continue in the merge process of the actual PR to ArduinoJson

[andreaskuster]

@bblanchon: See #1562 (comment)

[bblanchon]

Hi @andreaskuster,

Thank you very much for this PR ❤️

Supposing that the build passes, I'll make a few changes to make it more consistent with the other jobs.
Please don't feel offended if I modify your code: it's not a judgment about the PR; it's just me being pedantic.

Best regards,
Benoit

[coveralls]

Coverage remained the same at 99.044% when pulling d918b43 on andreaskuster:master into a6fc931 on bblanchon:6.x.

[andreaskuster]

@bblanchon Sure, not problem. I am glad to help :)

It seems as if the ci pipeline passed (except some unrelated package download in GCC for ARM processor

Let me know if you need any help or explanation.