Refactor maybe_create_object_store_from_uri

[irenedea]

What does this PR do?

Refactors maybe_create_object_store_from_uri to use a dictionary, which maps backend strings to object store factory functions.

What issue(s) does this change relate to?

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[mvpatel2000]

Should we add a test for registering a mock option?

[irenedea]

@mvpatel2000 The existing test test_maybe_create_object_store_from_uri already tests that items in the dictionary are read from. I think that should be sufficient to handle the case where someone adds a new items to the dictionary.

[mvpatel2000]

LGTM. not sure why comments are removed though