Asset sha hashing

[piercefreeman]

We should support server-side invalidation of client style files while still maximizing cache hits.

When importing local dependencies with a LinkAttribute, we should hash the contents at build-time so we can reference a dynamic sha in the injected URL. This will require:

• Traverse any mounted StaticFiles directories. Even if this is overridden by nginx at host-time we assume users will have mounted this programmatically during development.
• Cache a new artifact file at build-time that maps the filename to the current sha
• When a LinkAttribute is created at runtime (for either global scope or dynamic render scope) we will do a lookup of the global artifact and determine if we have a sha referenced. If so, we'll include it as a ?v={sha} query param.
• This should be opt-out with a keyword param during LinkAttribute creation