Let non-hierarchical links to hierachical elements be relative paths for self-contained catalogs

[jpolchlo]

Related Issue(s):
Closes #1112

Description:
This is an attempt to address the unexpected and maybe undesirable behavior identified in #1112. For this, we consider the case of a catalog tree which is defined using hierarchical links. When serialized out as a self-contained catalog, using to_dict yields relative links for all elements. Now, when we add additional links that have non-hierarchical rel types, but refer to entities which are in the hierarchical tree, the non-standard links are serialized with absolute hrefs.

For the following tree

flowchart TD
    root -- child --> A
    root --child--> B
    A -. sibling .-> B

Loading

When we serialize A, we want to see

{
    "type": "Catalog",
    "id": "a",
    "stac_version": "1.0.0",
    "description": "a",
    "links": [
        {
            "rel": "root",
            "href": "../catalog.json",
            "type": "application/json"
        },
        {
            "rel": "sibling",
            "href": "../b/catalog.json"
        },
        {
            "rel": "parent",
            "href": "../catalog.json",
            "type": "application/json"
        },
        {
            "rel": "self",
            "href": "/opt/data/stac/a/catalog.json",
            "type": "application/json"
        }
    ]
}

because the referent of the sibling link has been serialized elsewhere in the catalog tree using a relative link. This is the result that is delivered by this PR. Whether this should be the default behavior is a separate concern, and we can have that discussion below.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov]

Codecov Report

Patch coverage: 95.23% and project coverage change: +0.01 🎉

Comparison is base (ba74fd5) 91.99% compared to head (27b733c) 92.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1169      +/-   ##
==========================================
+ Coverage   91.99%   92.00%   +0.01%     
==========================================
  Files          51       51              
  Lines        6792     6808      +16     
  Branches     1000     1007       +7     
==========================================
+ Hits         6248     6264      +16     
  Misses        368      368              
  Partials      176      176              

Impacted Files	Coverage Δ
pystac/link.py	91.54% <80.00%> (+0.04%)	⬆️
pystac/stac_object.py	95.13% <100.00%> (+0.34%)	⬆️
pystac/validation/local_validator.py	95.23% <100.00%> (ø)

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[gadomski]

Can you add a test case that mirrors the example in the PR description? Thanks!