Clarify the form and constraints of target_url

OTIO has long used `target_url` as a way to refer to assets referenced by a timeline (such as media files). However the ruleset for how these URLs are formatted has been "wild west" since the fields to store them are strings.

We should explicitly specify how these urls are formatted. Considerations include:

- Convention about URL scheme formatting
- Clarifications and examples for the most common use case of `file://` urls
- Clarifications about [percent-encoding](https://en.wikipedia.org/wiki/Percent-encoding)
- Explicit specification about how to resolve relative URLs to a resource

This work should likely be followed up with some code to assist users in properly constructing and working with these URLs for the common use cases.

No files, tests, or entry points are named. Review the existing OTIO specification for target_url and compare its treatment of URL schemes, file:// URLs, percent-encoding, and relative resolution; done means those rules are explicitly specified with common examples and follow-up implementation needs are identified.

Written by the indexing model from the issue text.