Angular: Port Firebase executor from nxtend

[dolphinigle]

According to this comment, nxtend is being deprecated in favor of nxext. But Nxext doesn't currently support firebase, unlike nxtend.

It would be great if Nxext supports firebase too.

If this is within the scope of Nxext, I am happy to contribute by porting this feature with a PR.

[DominikPieper]

Hey,
right now we don't plan to migrate @nxtend/firebase to Nxext.

[WhatsThatItsPat]

Alternatively, perhaps Nxext would be a good place for a successor to https://github.com/simondotm/nx-firebase