fix array_agg to work with complex types and bugs with expression aggregator complex array handling

[clintropolis]

Description

This PR removes an overly restrictive validation check on ARRAY_AGG which was preventing use with druid COMPLEX types, such as COMPLEX<json>. This check is a relic from when this function was added, which was before the native expression system supported COMPLEX types.

After removing this check, I ran into a couple of rough edges, and a bug in ObjectStrategyComplexTypeStrategy, which was missing a check to ensure that the positional read method actually moved the buffer position forward by the number of bytes read (useful for array_agg which is serializing arrays) instead of relying on the underlying ObjectStrategy to move the buffer.

After the bugs, the result seems pretty useful, particularly for COMPLEX<json> (though certainly not the speediest thing around since its implemented as an expression aggregator):

even distinct works:

While here, I also fixed a bug with the output type inference of ARRAY_OFFSET and ARRAY_ORDINAL functions, as well as made native ARRAY typed expressions more consistently honor druid.generic.useDefaultValueForNull.

This PR has:

• been self-reviewed.
• a release note entry in the PR description.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.