[CX] specifying scalar functions that take arrays as input

There are a number of functions (not public in CBAS though, https://ci.apache.org/projects/asterixdb/sqlpp/builtins.html) that take an array as one of their input parameters.

For example:
string_concat
string_join
codepoint_to_string
binary_concat
create_polygon
similarity_jaccard
similarity_jaccard_check
object_remove_fields
object_add_fields
len
deep_equal

We need to review those functions carefully:
1. Are those functions also aggregate functions (in principle they are)? Should they work in group-by/aggregations (need testing and verifying)?
2. How should null/missing be handled when they appear in an input array parameter?