Refactor get_available_variable_names to return a span<string> rather than a concrete vector<string>

[PhilMiller]

This allows implementations to vary how they store and provide the available names (to some extent), potentially cutting memory overhead.

Changes

• Change DataProvider interface and all implementations to return span<string>
• Use auto at call sites, since they uniformly don't actually care that they've gotten a vector, as opposed to a find()-able collection

Notes

• Inspired by this comment: https://github.com/NOAA-OWP/ngen/pull/601/files#r1330439844 which marks a spot that is now changed in tandem

Checklist

• PR has an informative and human-readable title
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• Any change in functionality is tested
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• Placeholder code is flagged / future todos are captured in comments
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[PhilMiller]

The broader philosophical goal of this change and others like it is to make it so that wherever possible, the code deals in terms of cheap to create/copy/manipulate value types such as span, rather than types that require careful consideration of how to handle them at every place in the code that they're used. In particular, having a vector-based return type invites mistakes of accidentally picking it up by value and copying it, along with all its contents.

[PhilMiller]

With the merge of #601, this has been expanded to spare the memory footprint cost of an empty vector for every instance of NullForcingProvider int he realization configuration.