providers/heroku: add heroku_app_feature resource

[danp]

All credit to @bernerdschaefer, just PR'ing something he did internally.

Also, 👋 @catsby!

[catsby]

LGTM, thanks!

TF_ACC=1 go test ./builtin/providers/heroku -v -run=TestAccHerokuAppFeature -timeout 120m
=== RUN   TestAccHerokuAppFeature
--- PASS: TestAccHerokuAppFeature (6.97s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/heroku 6.982s

[catsby]

@danp ah, I merged this before I noticed the new resource wasn't in the layout! I added that in d6560b9 and had to do a little hack on the heroku_app docs to get the regex to only highlight one or the other on our docs page. The regex hack is a blemish but it works for now 😄

[danp]

Oh, thanks! Will be sure to update that list too next time.

[ghost]

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