[Variant] support `..` and `['fieldName']` syntax in the VariantPath parser

[alamb]

Interesting. It looks like the mini-parser I posted above is a strict subset of the databricks syntax linked above. Would it be a reasonable starting point to merge this PR and add the missing ['fieldName'] syntax as a follow-up? Or just expand the above suggestion into a full parser right away?

Originally posted by @scovich in #9012 (comment)

[klion26]

take