Add Merge-DevPullRequest cmdlet and update documentation

[isra-fel]

• Introduced Merge-DevPullRequest cmdlet to facilitate merging pull requests in the azure-powershell repository.
• Updated README.md to include usage instructions for the new cmdlet.
• Added entry to CHANGELOG.md for the new feature.
• Modified AzDev.psd1 to export the new cmdlet and its alias.

Description

Mandatory Checklist

• Please choose the target release of Azure PowerShell. (⚠️Target release is a different concept from API readiness. Please click below links for details.)

  • [ ] General release
  • [ ] Public preview
  • [ ] Private preview
  • [ ] Engineering build
  • [x] No need for a release

• [x] Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD update ChangeLog.md file(s) appropriately
  • Update src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md.
    • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT adjust version of module manually in pull request

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