Adding new team sigstore-a2a

[SequeI]

Summary

Adding teams for model-validation-operator and sigstore-a2a. Second pull request will add users to these teams.

cc @miyunari

EDIT: Just saw MVO has already been added, rebased PR to remove MVO

Release Note

Documentation

[github-actions]

🍹 preview on sigstore-github-sync/sigstore/github-prod

Pulumi report

View in Pulumi Cloud

  Previewing update (github-prod)

View Live: https://app.pulumi.com/sigstore/sigstore-github-sync/github-prod/previews/151e6338-66e3-445b-b252-96a2257dd8c7

@ Previewing update.....
pulumi:pulumi:Stack: (same)
  [urn=urn:pulumi:github-prod::sigstore-github-sync::pulumi:pulumi:Stack::sigstore-github-sync-github-prod]
  + github:index/team:Team: (create) 🔒
      [urn=urn:pulumi:github-prod::sigstore-github-sync::github:index/team:Team::sigstore-a2a-codeowners]
      [provider=urn:pulumi:github-prod::sigstore-github-sync::pulumi:providers:github::default_6_9_1::2c59db6b-6387-45d1-b67c-4600e7fcbe4f]
      createDefaultMaintainer: false
      description            : "Codeowners for sigstore/sigstore-a2a"
      name                   : "sigstore-a2a-codeowners"
      parentTeamId           : ""
      privacy                : "closed"
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Resources:
  + 1 to create
  626 unchanged