allow specifying shards for HasManyAssociation#with_each_shard
allow specifying shards for HasManyAssociation#with_each_shard

this stub doesn't do anything with them, but specific sharding
implementations can use them, and the signature should allow that.

Change-Id: I11a27e52e1e98e7aa77818dcf5a00d829bdaef41
Reviewed-on: https://gerrit.instructure.com/17578
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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