refactor link relation tables, make parent not a required field

[philvarner]

Related Issue(s):

• parent link is not required for any object #406

Proposed Changes:

1. parent link rel is not required for Collection or Item. In <= 1.0.0-rc.2, it was required for /collections/{collectionId}, and the requirement on /collections/{collectionId}/items/{itemId} was added since 1.0.0-rc.2. This aligns with it not being required in STAC Item and Collection definitions.
2. Link relation tables are refactored to align with the style in stac-spec of using REQUIRED for required relations.

PR Checklist:

• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[tschaub]

Looks great, @philvarner. Thanks for fixing this up.

[m-mohr]

I'm actually not sure whether we should drop the requirement. I feel like requiring the parent links (and potentially even more) would help in the long run. I think adding links in API implementations is much "cheaper" than in static catalogs so I'd think we could require more in the API compared to static catalogs.

[tschaub]

Maybe conditional requirements are too sloppy, but how about only requiring parent if an Item is not part of a Collection? I've only worked with Items whose "parent" is a Collection, so I'm not sure how common Collection-less Items are in STAC API implementations. It just struck me as odd to require that /collections/{collectionId}/items/{itemId} link to /collections/{collectionId} twice (once as parent and once as collection).

[m-mohr]

That's weird as you'd need a condition in implementation, but I also don't think having them twice is an issue.
The same may occur in the collection where root and parent are pointing to the same thing.
It's pretty common in static STAC (although not required) so I feel we could at least recommend all of the links (self, root, parent, collection)?

[philvarner]

I think if we want to keep Collection parent as required and have Item parent as required (since it was inadvertently added by me without discussion), we can do that, but I'd prefer we have that as a future discussion for 1.0.0-rc.4. I prefer aligning them with STAC definitions for now. I really just want to get 1.0.0-rc.3 out.