Add export-section command to CodeownersTool and CODEOWNERS section change detection to codeowners-linter pipeline

[Copilot]

Two related changes to enable detection of modifications to protected CODEOWNERS sections in PRs.

azsdk config codeowners export-section (Issue 1)

New CLI-only subcommand under CodeownersTool that reads one or more named sections from a CODEOWNERS file and writes their content to a single output file. --section is required and multi-value; missing sections log an error and exit 1.

azsdk config codeowners export-section \
  --codeowners-path .github/CODEOWNERS \
  --section "Client Libraries" \
  --output /tmp/sections.txt

Uses the existing CodeownersSectionFinder.FindSection utility.

codeowners-linter.yml section change detection (Issue 2)

Inside the existing PR-only block:

• Installs azsdk via install-azsdk-cli.yml (always latest)
• Exports the Client Libraries section on the base branch → $(Build.ArtifactStagingDirectory)/codeowners-sections/before.txt
• Exports the same section on the PR branch → .../after.txt
• Fails the build if the two files differ

No new pipeline parameters; section names are hardcoded in the step commands.

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