Use KSQL types in UD(A)Fs instead of BigDecimal/List/Struct

[agavra]

In order to give more flexibility and type safety to implementations of UDFs, we should do the following:

1. wrap BigDecimal in KsqlDecimal which will provide both the schema and the value
2. wrap List in KsqlArray which will provide both the schema and the value
3. do not use Struct in udfs, but use KsqlStruct instead

[big-andy-coates]

And of course, move to using SqlType and removing use of Connect's Schema in UDFS

[big-andy-coates]

Removing out dependency on the Connect schema and types for our UDF framework (and serde framework!) will fix a whole family of bugs. See below for linked bugs as we find them.