WIP: Support transaction in the docstore

[sandeepvinayak]

docstore/all

Details in the issue : #3501

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[sandeepvinayak]

@eliben @vangent We are trying to add the transactions support/atomic writes in the docstore and this is the high level idea of how can we support that in go-cloud. https://docs.google.com/document/d/1UVj1kmwDfrs5qm8r7X1p4fAFmsdPEeBHcvWJ8zWF1dY/edit?tab=t.0

The PR is just at a high level to support the idea.
Let me know what you think about it and we can help to drive it further.