Require KEY column in C* statements

[big-andy-coates]

Currently, if the user does not provide a key column in their stream definition, e.g.

CREATE STREAM FOO (COL0 INT, COL1 STRING) WITH (...);

Then ksqlDB adds a default ROWKEY STRING KEY column to the schema of the stream.

We should remove this magic and instead require the user to always supply the key column.

[big-andy-coates]

To achieve this, we first need key schema inference #4462. Because we don't want to have to force people using Avro key+values to have to define their key schema in C* statements.

[big-andy-coates]

We should do #3681 before this as well, to ensure we only require users to make one change.

[big-andy-coates]

Closed in favour of just going with #4678