extract information from STAC extensions

[keewis]

xpystac used to rely mainly on the xarray STAC extension to infer any additional parameters needed to open the data. That extension has been superseded by the storage and auth extensions, and additionally there are now additional extensions like the alternate extension with additional ways to describe metadata needed to open the data.

This mainly adds infrastructure to extract the metadata, there still needs to be a translation layer that converts to xarray options.

Checklist

• Linting: Code is formatted and linted
• Tests: Tests pass. I have included new tests for these changes where applicable.
• Edge Cases: I have manually verified "unhappy paths" and edge cases beyond the basic success criteria (e.g., database connection timeouts, malformed input, strict mapping rejections).
• Documentation: I have updated README.md to reflect any new environment variables, configuration changes, or breaking updates.
• Accountability: I can explain the implementation logic for every line of code submitted.

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AI tool usage

none, all hand-written.

[jsignell]

Thanks for opening this @keewis! I like the separate modules a lot.

[jsignell]

Having started looking through this a bit I'm wondering if this could be an opportunity to use pystac extension classes rather than parsing the json.

[keewis]

most likely. I had a look at this when I implemented this a couple of months ago but decided it was too complicated at the time: AFAIR a alternate asset extension didn't exist back then (I didn't check if it does now), and implementing it myself took more than the little mental capacity I had at the time.

Once that exists, though, I'd be happy to change the PR.