KVs should not be needed when using schemas

When using schemas there should be no need to require a KV, as any. key or keys can be act as the K or a V in a KV. Users can use schemas without the need for KV in all cases except for state/timers DoFns, which today require that the input have type KV. We would like to eliminate this requirement. Users should be able to specify a key field extraction in their ParDo, e.g.

ParDo.of(...).withKeyFields("user.location.city"));

Or in the DoFn itself.

Imported from Jira [BEAM-12795](https://issues.apache.org/jira/browse/BEAM-12795). Original Jira may contain additional context.
Reported by: reuvenlax.