Annotate paths with a version changelog

[cmars]

API documentation needs to represent a "changelog" per resource endpoint: a list of versions in which a change to a given resource endpoint was released.

This can be represented as an OpenAPI extension on the PathInfo element. For example, a response to GET /openapi/experimental might show:

...
paths:
  /some_resource:
...
    x-snyk-api-version: '2021-08-09~experimental'
    x-snyk-api-releases: ['2021-06-07~experimental', '2021-07-21~beta', '2021-08-09~experimental']

Indicating that /some_resource has 3 versioned releases. The latest experimental is the one shown, but if one wanted to compare this against prior changes, they could look at 2021-06-07~experimental or 2021-07-21~beta.

To implement this (CC @mrded @jcsackett), I would look at adding the extensions to paths in vervet.LoadResourceVersionsFileset. After the sort, you'll have everything you need:

• The OpenAPI specs at each resource version relevant to this particular resource.
• Sorted in version order, ascending.

The dead-simplest approach would be to set x-snyk-api-releases to a slice containing this list of versions, at each version. That might be slightly redundant in cases where the only release change was an endpoint being added or removed, since this list of versions is per-resource rather than per-endpoint. In practice, that may not matter much -- it could be something to refine later, or we might actually want to highlight that the resource changed, while this particular endpoint did not.

[cmars]

Fixed in #89