fix complex_decode_base64 function, add SQL bindings

[clintropolis]

Description

Fixes some bugs with the complex_decode_base64 function, which was basically broken before this PR for a few reasons, and migrates it to being implemented as an ExprMacro instead of a Function, so that the literal arguments can be computed externally to the function instead of every evaluation. This involves a slight adjustment to ExprMacroTable to add the concept of built-ins to make it a bit easier to add macros.

This PR also wires complex_decode_base64 up to SQL, previously it was only a native expression. I have purposefully still not documented this function yet to save room for future adjustments and determining the best way to position this functionality, since its use cases are currently a bit limited.

The improvements do allow some pretty funny/absurd things to work now, such as round-tripping complex types through JSON serialization and back into complex types all in the same expression as shown in one of the tests added:

SELECT APPROX_COUNT_DISTINCT_BUILTIN(COMPLEX_DECODE_BASE64('hyperUnique',PARSE_JSON(TO_JSON_STRING(unique_dim1)))) from druid.foo

😜

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This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• 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.

[rohangarg]

Thanks a lot for the changes @clintropolis ! LGTM % comments