Add a views argument validator plugin for project nids which takes nids or project short names

The solution to a few of the problems at #76725-43: Refactor project issue module to use Views is to have a views plugin that can validate project nid arguments which understands either numeric nids or project short names (uri). It can also specify that the title for this argument should be the full project title. Although this is needed initially for project_issue, I think it makes the most sense to add it to project's views directory, since it's generic enough that other parts of project* might need it.

Any objections?