(Part 2) Missing requirement for link to collections

[jerstlouis]

See https://github.com/opengeospatial/ogcapi-code-sprint-2021-07/issues/22

Suggesting:

The OGC API landing page response links SHALL include a link to the set of available collections (at /collections) using the link relation type http://www.opengis.net/def/rel/ogc/1.0/data.

[cmheazel]

The data link relation type has been added to Table 1 in Part 2
Part 2 Section 8.1 identifies the data link relation type as another way to access the Collections resource.
I don't believe we can make use of data as a requirement because:

1. It's an OGC-specific link relation type
2. Part 2 does not require the Landing Page conformance class so it cannot have requirements that assume that it has been implemented.
3. Part 1 does not require Part 2 so it cannot impose requirements that assume Part 2 has been implemented.

[jerstlouis]

@cmheazel with modular building blocks, I think at one point requirements would need to be conditional so as to define how they connect to each other, e.g. (as a Part 2 requirement):

If the API also declares conformance with OGC API - Common - Part 1, then the OGC API landing page response links SHALL include a link to the set of available collections (at /collections) using the link relation type http://www.opengis.net/def/rel/ogc/1.0/data.

That would still be a testable requirement.

[cmheazel]

See issues #132 and #294

Issue #294 added part B (below) to Requirement 2 (GET /collections).
A - The API SHALL support the HTTP GET operation at the path /collections.
B - The API SHALL support the HTTP GET operation on all links to a Collections Resource that have the relation type http://www.opengis.net/def/rel/ogc/1.0/data.

While Part 2 cannot levy requirements on resources defined in Part 1, it can require how a Part 2 resourced SHALL respond if referenced using the "data" relation type. The origin of the relation doesn't matter. The behavior of the API is defined based on the relation type and target resource type.

[jerstlouis]

Suggesting for B instead:

B) If the API has a mechanism for exposing root resources (e.g., a landing page), the API SHALL advertise at least one URI to retrieve the list of collections provided by this service with a link having a rel value: http://www.opengis.net/def/rel/ogc/1.0/data.

EDITED: (I had misread). It is a bit confusing to talk about GET operations on links rather than resources.

See mode detailed explanation in #294 (comment) (including similar example from Tiles, and conditional requirements & ModSpec/metanorma in metanorma/metanorma-ogc#439)