Support "literal expressions" for pull queries on advanced keys

[agavra]

We will soon enable support for structured and other advanced key types (map/array) (#824) but we will not be able to issue pull queries against materialized views that have those as keys. There are two tasks that need to be done in that bucket:

1. support the ability to create structs/maps/arrays in the pull query's where clause (e.g. WHERE rowkey = STRUCT(foo:=bar)) so long as every element within the struct/array/map declaration is a literal (or resolvable to a literal value)
2. support the ability to issue pull queries against multi-column keys (e.g. WHERE colA = 'foo' AND colB = 'bar').

This ticket tracks the former of the two.