[Feature] Build using esp8266/Arduino SHA as platform

[TD-er]

As discussed here: esp8266/Arduino#5527 (comment)
It would be nice to have some easy method to define a version of the Arduino repo as platform to make it easier to test fixes in the Arduino branch.

For example either by using a PR number or a SHA of a specific commit or branch.
It should be as easy as switching platform like the current stage platform definition.

Things to consider here:

• version number in the JSON differs from the ones used in PlatformIO
• Limit to a branch/commit/PR in the esp8266/Arduino repo or maybe also a branch/repo of a developer?