Add LazilyOuterIndexedArray alias to LazilyIndexedArray for external backends to use

[alexamici]

• Closes Rename of LazilyOuterIndexedArray to LazilyIndexedArray breaks rioxarray #5111
• No need to add tests
• Passes pre-commit run --all-files

[mathause]

Will the alias be kept or should you add a deprecation warning?

[alexamici]

I see quite a few uses "in the wild":

https://github.com/search?l=Python&q=LazilyOuterIndexedArray&type=Code

I would suggest to keep it long term to avoid migration pains to external backend developers.

[alexamici]

If nobody objects I'll merge this shortly as this breaks building against master of external projects testing out the backend-plugin infrastructure. We can always add a deprecation warning on a follow-up PR.