Add `outputHashMode` argument attribute to bulitins that could support it

[Ericson2314]

An output outputHashMode argument attribute should be supported by all of

• builtins.derivation
• builtins.path
• builtins.fetchTree
• builtins.fetchurl

In order to specify which ContentAddressMethod is used for creating store paths.

Misc notes:

• For builtins.derivation this is specifying that for a content addressed (fixed or floating) derivation output, for the other two, it is specifying that for the store path the builtin creates directly.

• With lazy trees, builtins.fetchTree may not longer create a path. Perhaps paths created from it still turn into store objects, and it would affect those. Or maybe it's just better to separate concerns and not have this argument on builtins.fetchTree after all.

[roberth]

• builtins.fetchTree

Not sure.
If we factor it to be just the fetching, the hash mode is not relevant until we fetchToStore, ie builtins.path or interpolation.
Or should path values carry a preferred output hash mode? I doubt that, but maybe?

• builtins.path

Possible use case:

• In Disallow output changes in installCheckPhase nixpkgs#303361, I originally wanted to fetch just setup.sh directly from the API with fetchurl, which didn't give me a text-hashed path, resulting in a mass-rebuild (and the whole point was to avoid that). I tried to convert the path with builtins.path, but it didn't let me switch to text hashing. If I didn't control the fetchurl call, this would have been my only solution. However in general, let's add:

• builtins.fetchurl

[Ericson2314]

Good point with builtins.freeTree.