`variant_length`: Initial implementation

[sdf-jkl]

follow up to #32

variant_length(VariantArray, path)

https://github.com/datafusion-contrib/datafusion-functions-json/blob/main/src/json_length.rs

Returns the length of an Array or Object at the given path.

Arguments

• VariantArray -- a list of variant values
• path -- the variant path string

Returns

• Uint64 (Nullable)