Provide SPI to override schema and value conversion for specific columns

There are frequent requests in the form "Can't a column of type ABC not be emitted as Connect type xyz instead of what Debezium currently export?". E.g. this comes up often for temporal column types, with the intention being to export them as a formatted string.

The standard answer to that so far has been "Use an SMT to implement the conversion". But we may consider to establish an SPI for this, which makes custom schema definers and value converters possible. The prime motivation being the avoid the performance penalty of applying the conversion of schema and type after the fact.