rpc: retrieve unresolved transactions

[harshit-gangal]

Description

This PR adds a new vttablet rpc to retrieve unresolved distributed transactions from the primary tablet.

Related Issue(s)

• RFC: Atomic Distributed Transactions #16245

Checklist

• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
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Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

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Backward compatibility

• Protobuf changes should be wire-compatible.
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