Expand support for multiple extension uris

[gadomski]

In #1081 we added support for older schema URIs to the ProjectionExtension. This behavior should be generalized so other extensions can use it easily. Ideas:

• Add get_schema_uris() to the base mixin
• Refactor the base has_extension() to check against get_schema_uris() instead of get_schema_uri()
• Backport any extensions that have older-but-compatible schema uris (thinking maybe label at least?)

This doesn't solve the problem, it's more of a band-aid -- if a newer version of an extension is incompatible with the Python structure, we don't have a mechanism to "migrate".

xref #448 which is our tracking issue for the general problem of extension versions.