Duplicate ShardedKey implementations in core module

wo distinct ShardedKey classes exist in the Java SDK core module with overlapping semantics:org.apache.beam.sdk.values.ShardedKey,org.apache.beam.sdk.util.ShardedKey.The only difference is the shard ID type (int vs byte[]), while the core semantics are identical. This duplication creates confusion for users and increases the risk of errors (e.g., accidental import of the wrong class).