feat: Add example raft-kv-memstore-opendal-snapshot-data

[Xuanwo]

As title.

Checklist

• Updated guide with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done.
• Unittest is a friend:)

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This change is 

[Xuanwo]

cc @drmingdrmer, @schreter, would you like to take a review? Thanks!

[drmingdrmer]

Reviewed 9 of 14 files at r1, all commit messages.
Reviewable status: 9 of 14 files reviewed, 1 unresolved discussion (waiting on @Xuanwo)

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examples/raft-kv-memstore-opendal-snapshot-data/README.md line 1 at r1 (raw file):

# Example Openraft kv-store with snapshot stored in remote storage

Explain why you create this example

[Xuanwo]

The failure of examples (nightly, raft-kv-rocksdb) seems not related to this PR?

[drmingdrmer]

The failure of examples (nightly, raft-kv-rocksdb) seems not related to this PR?

Right, it fails frequently. Ignore it.

[drmingdrmer]

Reviewed 4 of 14 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @Xuanwo)