Implement changing topic replication factor via AlterPartitionAssignments requests

[graphcareful]

Currently our implementation of AlterPartitionAssignments does not accept requests that attempt to modify assignments in a way that would increase (or decrease) the topic replication factor. This however is not how Kafka implements this request.

To implement this feature we would need to modify some cluster level APIs that will modify reassignments for an entire topic because currently the only support for this is move_partition_replicas which is more lower level. It reassigns individual partitions.

Then we would need to explicitly support lowering the replication factor of a topic, a feature that does not exist in redpanda at this time.

JIRA Link: CORE-1156