Add CODEOWNERS to the acir serialization codegen file

[kevaundray]

Problem

If the serialization changes, then its a breaking change that breaks every backend. Tom pointed out that assigning codeowners would be good so that these changes do not get past code review by one of said code owners.

Comment: #2961 (comment)

Happy Case

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Alternatives Considered

No response

Additional Context

No response

Would you like to submit a PR for this Issue?

No

Support Needs

No response

[kevaundray]

Closing until the need for this pops up due to a lot of people editing the file and not knowing what the repercussions are