feat: support exp manager checkpointing with object store via Multi-Storage Client

[jayya2]

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What does this PR do ?

Add support for using multi-storage-client for the Exp Manager checkpointing mechanism so that users who haven't migrated to new checkpoints can still take advantage of the object stores.

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[titu1994]

Overall ok, but the code is getting super complicated.
Can you avoid trying to add is_msc_* to every single check and simply separate the code paths to be completely independent of each other if msc is there or not?

Right now this mix match of merging msc logic with older logic makes the code unreadable.

[jayya2]

@titu1994
thanks for the quick review, I addressed most of the comments:

• Have isolated functions to wrap up MSC calls to prevent a mix of logic
• Reduce the number of MSC url checks
• Use full name for msc (after second thought I don't want to mix both multiplestorageclient and msc so I would rather have long name everywhere)

Let me know if further refactoring is required, thanks!

[titu1994]

Much cleaner than before, thank you !

[ko3n1g]

@jayya2 @titu1994 : This PR got affected by multiple infra issues. Please don't relabel this PR, we will restart failed jobs for you and try to get this merged with as little overhead as possible. TIA!

[jayya2]

@ericharper @titu1994 @ko3n1g are the infra issues resolved? saw some of the tests retried and passed but still 2 failing but seems unrelated to this change

[ericharper]

Re-approving