Support schema map for validation

[philvarner]

Currently, the validation code always retrieves the remove STAC schema URI as advertised in the item. However, this can cause significant performance and transient failure issues when creating and validating items. One fix for this is to (as stac-validator does) allow the caller to supply a map of schema URIs to local files, so it's a local read operation rather than a remote network call. Overall, I think we should attempt to move some of the validation behavior from stac-validator into this package, so that stac-validator can focus more on just being a CLI shell around pystac.

[gadomski]

Currently, the validation code always retrieves the remove STAC schema URI as advertised in the item.

Not true for the core schemas: https://github.com/stac-utils/pystac/blob/main/pystac/validation/local_validator.py

Overall, I think we should attempt to move some of the validation behavior from stac-validator into this package, so that stac-validator can focus more on just being a CLI shell around pystac.

Agreed.

[jsignell]

Another option would be to cache the files on the first download. Since extension links point to released versions it seems like you'd never really have to even expire such a cache.