[PFS-207] Add GetWithID Functions to Projects and Repos

[FahadBSyed]

The goal of this PR is to add GetXWithID functions to Projects and Repos so they can be consumed by PFS APIs.

[brendoncarroll]

Check if you need the double pointer here.

[FahadBSyed]

This is actually only one pointer:

// ProjectNotFoundError is returned by GetProject() when a project is not found in postgres.
type ProjectNotFoundError struct {
	Name string
	ID   ProjectID
}

But to confirm, that this works, we're using it in projects_test.go

...
		// validate error for attempting to get non-existent project.
		_, err = pfsdb.GetProject(cbCtx, tx, 3)
		require.YesError(t, err, "should not be able to get non-existent project")
		require.True(t, (&pfsdb.ProjectNotFoundError{ID: 3}).Is(err))
		return nil
	}))

[brendoncarroll]

A few things. Otherwise LGTM.

errors.As is tricky. I would exercise that code path in a test.