[refactor] Move cassandra implementation to `internal` package #1431

Job	Run time
badger / badger (v1)	1m 59s
badger / badger (v2)	4m 2s
cassandra / cassandra-4.x v1 schema=manual	5m 51s
cassandra / cassandra-5.x v1 schema=manual	6m 0s
cassandra / cassandra-4.x v2 schema=manual	7m 46s
elasticsearch / elasticsearch 6.x v1	5m 35s
cassandra / cassandra-5.x v2 schema=manual	7m 53s
cassandra / cassandra-4.x v2 schema=auto	7m 45s
elasticsearch / elasticsearch 7.x v1	5m 59s
cassandra / cassandra-5.x v2 schema=auto	7m 49s
elasticsearch / elasticsearch 8.x v1	6m 0s
elasticsearch / elasticsearch 8.x v2	6m 26s
grpc / grpc (v1)	2m 10s
kafka / kafka 3.x v1	2m 34s
grpc / grpc (v2)	4m 21s
kafka / kafka 3.x v2	5m 6s
memory / memory-v2	3m 54s
opensearch / opensearch 1.x v1	5m 42s
opensearch / opensearch 2.x v1	5m 57s
opensearch / opensearch 2.x v2	6m 14s
	1h 49m 3s

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