implement build-url and build-url-short

[norbertbuchmueller]

One often uses the metadata envvars to construct a URL to the Concourse build page. This PR makes it easier - by moving the logic of that into this resource.

[norbertbuchmueller]

@regevbr what's your opinion of the proposed changes?

[regevbr]

@norbertbuchmueller thanks for the contribution and Sorry for the delay,

I don't mind adding those "variables" but please refactor the code so you won't have to hack the values into the environment - create a method to extract the env values for the existing variables and pass the values directly to the handle prop method, then in your props, just pass the values without calling the helper method.

Also please make sure the tests pass after you make the changes.

@ezraroi what do you think?