Add templated-url format to the registry

[MikeRalphson]

As discussed with @jdesrosiers on Slack

[handrews]

(pardon muddled thoughts - I'm recovering from COVID) is there a reason to not make this oas-templated-url? There might be a wide range of formats that fit the vague description here, making the format difficult to support.

[MikeRalphson]

There's no reason not to, is it just for clarity? AsyncAPI might use something very similar though.

[handrews]

trying to avoid a repeat of decimal where there are different meanings depending on context

[handrews]

i think the question to ask is: what guarantees can tools assume/implement when they see this format? Currently, not much. URI + { and }, but that is a broader constraint than OAS defines - OAS puts constraints on what parts of a URI can be replaced by a variable, and might constrain the variable syntax (I can't recall)

[handrews]

See also #3256 (Formally define templating behavior)

[handrews]

I think if we make this oas3-templated-url we could merge it. That avoids confusion with other URL templating systems, of which there are many, including ones that differ across OAS versions.

[handrews]

Also, I think this needs rebasing.

[handrews]

Since Mike's not available to rebase and discuss this, and we're already tracking the need for a format for this in OAI/spec.openapis.org#132 (which already links to this PR), I'm just going to close this. When someone is available to figure it out they can submit a new PR.