methods to create and parse standard resource prefixes with `repo_name`, `directory_path` as parameters

[tswast]

As discussed in Yoshi Python Chat and googleapis/python-bigquery#590 (comment)

It'd be good to have some utilities for constructing prefixes for resources based on the repo name and directory name, relative to repo root. This would include a timestamp (so that resources without a created time property could be aged) as well as a random identifier.

Likewise, we could have helpers to parse resource identifiers and determine (1) they match the repo name and directory name and (2) they are older than some time-to-live (24 hours seems a good default)

[tswast]

I actually realized my first implementation has a problem, which I believe I've fixed in googleapis/python-bigquery#627

Turns out that DatasetListItem objects don't actually have a created property (even though Dataset does), so it is more efficient to parse the date from the resource name.

On second glance at this, I'd probably remove the _ from the date format`.

[tswast]

Started prototyping here: https://github.com/googleapis/python-test-utils/tree/issue10-resource-prefixes

I don't think we'll want a whole bunch of extra arguments to resource_prefix() and the other functions. I think we'll want some kind of config. Maybe it's actually part of a class? That might actually be an appropriate use for one (holding the RESOURCE_PREFIX constant)