Drop requirement that CREATE TABLE must have KEY set in WITH clause.

[big-andy-coates]

The requirement is excluding any use case where the topic data does not have a copy of the key in the value. It is also confusing users. It is already possible to have tables in the metastore that does not have a key field, consider:

CREATE TABLE T1 (ID INT, OTHER INT) WITH (KEY='ID', ...);
CREATE TABLE T2 AS SELECT OTHER FROM T1l;

T2 will not have a key field set as the source's key field is not in the projection.

The KEY property should be an optional optimisation hint, that allows KSQL to avoid unnecessary repartitions