feat: added delivery format to the contract

[Cali0707]

Part of #3970

Proposed Changes

• Add the delivery format option to the data plane contract

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Cali0707

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[Cali0707]

/cc @Leo6Leo @matzew

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 48.16%. Comparing base (566cb7c) to head (ffe31e5).
Report is 3 commits behind head on main.

Files	Patch %	Lines
control-plane/pkg/contract/contract.pb.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3980      +/-   ##
==========================================
- Coverage   48.18%   48.16%   -0.02%     
==========================================
  Files         246      246              
  Lines       14518    14522       +4     
==========================================
  Hits         6995     6995              
- Misses       6817     6821       +4     
  Partials      706      706              

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[Leo6Leo]

/lgtm
As the number 8 is unique and not being reused in EgressConfig

[Cali0707]

/retest-required