Propose new maintainers for Cosign

[haydentherapper]

I am proposing adding Zach, Cody, and Colleen as Cosign maintainers. All have been active in Cosign's development and roadmap design. All have also been active in sigstore-go's development, and the convergence of sigstore-go and Cosign is a part of its roadmap.

Specifically:

• Zach has authored features on bundle support for binaries and conformance testing
• Cody has authored an implementation of the new bundle spec for attestations stored in OCI using Image Manifest v1.1 and proposed a similar spec for storing signatures as referring objects
• Colleen has authored numerous testing improvements and updates to Cosign's usage of TUF

Summary

Release Note

Documentation

[haydentherapper]

cc @hectorj2f @priyawadhwa @bobcallaway as current Cosign maintainers for approval

cc @steiza @codysoyland @cmurphy as proposed maintainers

[github-actions]

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

Pulumi report

  Previewing update (sigstore/github-prod)

View Live: https://app.pulumi.com/sigstore/sigstore-github-sync/github-prod/previews/3d022ac0-a351-4608-b20b-1d2bb5043dfd

@ Previewing update....
pulumi:pulumi:Stack: (same)
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@ Previewing update....
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      role      : "member"
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      role      : "member"
      teamId    : "4722092"
      username  : "steiza"
@ Previewing update....
Resources:
  + 3 to create
  568 unchanged