CASSANDRA-21169: Add compaction strategy override via system properties

[pauloricardomg]

Introduce the ability to override compaction strategy for specific keyspaces and tables at startup via two new system properties:

• cassandra.override_compaction.entities: comma-separated list of keyspaces and keyspace.table pairs (e.g. "ks1,ks2.tbl1,ks3.tbl2")
• cassandra.override_compaction.params: JSON compaction parameters to apply

Patch by Paulo Motta; Reviewed by X for CASSANDRA-21169

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