[v2] Refactor to standalone source HeadObject for copies

[hssyoo]

This PR refactors HeadObject calls during S3 to S3 multipart copies. Currently, the HeadObject request is called in SetMetadataDirectivePropsSubscriber. This isn't ideal since it couples HeadObject invocations to a specific subscriber. We want to open it up so that other subscribers can consume the HeadObject response, even if SetMetadataDirectivePropsSubscriber never fires.

Specifically, this refactor unblocks the following future changes:

• Update SetTagsSubscriber so it only fires if TagCount in HeadObject response is greater than 0.
• Hypothetical, but if we wanted to support more options for --copy-props (eg tagging) in the future, then these values can be fetched and copied independently from metadata.

[hssyoo]

Closing because the tag count optimization turns out to be infeasible.

The issue is that HeadObject will not return tag count at all under either conditions:

• Identity lacks permissions to get tags/tag count.
• The object doesn't have any tags.

Currently, S3 commands with --copy-props default will fail loudly if identity lacks tag permissions. With this optimization, it would start to fail silently, ie copy objects without tags since it doesn't see tag count in the HeadObject response. If HeadObject returned tag count = 0 when object doesn't have tags this would be possible but the current API doesn't give clients a way to distinguish between an object having 0 tags versus identity lacking tag permissions.