Define formats for URL templating with and without runtime expressions

[handrews]

PR #3234 proposes a url-template format, and is currently in discussion regarding the scope and naming of the format.

PR #3235 applied the format to the current schema, resulting in a debate over whether different formats are needed for URL templates that use runtime expressions vs those that do not.

There might also be other templating use cases in the Workflows spec that don't quite fit in either of these.

I'm filing this issue so we can track the various parts of the debate separately from the PRs, as PR #3235 overlaps with PR #3455 as well as having unresolved questions, so I am likely to close it.

Paging @MikeRalphson , @frankkilcommins

[handrews]

See also #3256 Formally define 3.x URL Templating behavior.