feat: add optional allow-list for fetching json schemas

[jsignell]

Related Issue(s):

• Add optional allow-list for domains in stac_extensions #1817

Description:

This PR adds an allow_list attribute on the Validator class. Not sure if this is worth putting into pystac itself or whether we should just show the pattern. Another option could be to provide a new Validator class that has the allow-list populated with some sane schema defaults. So that people could just do:

set_validator(SecureJsonSchemaSTACValidator)

as opposed to:

set_validator(
    JsonSchemaSTACValidator(
        allow_list=[
            "https://schemas.stacspec.org/",
            "https://stac-extensions.github.io/",
        ]
    )
)

PR Checklist:

• Pre-commit hooks pass (run pre-commit run --all-files)
• Tests pass (run pytest)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage
• This PR's title is formatted per conventional commits