feat: add COUNT_DISTINCT and allow generics in UDAFs

[agavra]

Description

Adds COUNT_DISTINCT to the ksqlDB UDAF suite with the following limitations:

• the syntax is note COUNT(DISTINCT <expr>) because we don't have support for this type of syntax yet
• the performance is likely bottlenecked by the throughput of array copies and auto/unboxing, I'm going to benchmark this to see if it's significant in the grand scheme of things (array copies of 2k ints may or may not be significant)
• the space overhead is (java Integer object size) 16 x 2731=43,696 bytes ~ 44kB per key

Also while making this change it looks like it was unnecessary to artificially restrict our UDAF framework from using generics.

Testing done

• unit testing
• QTT testing

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[MichaelDrogalis]

Nice one :)

[vpapavas]

Thank you @agavr , LGTM ! Excited that we support this functionality now and curious about the outcomes of the benchmark.