[Feature]: Rewrite ONTAP store as an S3 variant

Problem statement

ONTAP store implementation is meant to be mostly S3, but it's drifted and doesn't have all the improvements of the core S3 implementation.

Proposed solution

Following on from the discussion in https://github.com/TraceMachina/nativelink/issues/2767 and with https://github.com/TraceMachina/nativelink/pull/2768 now merged, we should be able to rewrite the ONTAP store as a thin layer over S3, similarly to the R2 and OCI stores.

The one difficult item here is access to a real ONTAP instance, which we don't currently have. The original ONTAP work was done in collaboration with a client with a local ONTAP store they could test things with, and rewriting this will need access to actual ONTAP. There does appear to be an ONTAP simulator available for download, but it looks like it needs a NetApp client license to do so.

Alternatives considered

Sticking with the existing ONTAP setup which is believed working (but again, lack of ONTAP makes it hard to test)

Additional context

No response

Read the R2 and OCI store implementations and the changes from merged PR #2768 first, then compare them with the existing ONTAP store. Done means ONTAP is a thin layer over the core S3 implementation and the result is validated against an actual ONTAP instance; arrange simulator or client access before relying on that validation.