chore(github): add CODEOWNERS

[UnbornAztecKing]

Summary

• Introduces CODEOWNERS to auto-assign reviewers and formalize ownership.
• Sets default owner to @dydxprotocol/client-code-owner for all files.
• Assigns @dydxprotocol/security as owner of .github/ and CODEOWNERS to enforce security oversight on policy/config changes.

Details

• GitHub/Infra:
  • New CODEOWNERS file:
    • Default: "*" → @dydxprotocol/client-code-owner.
    • Overrides: "/.github/" and "/CODEOWNERS" → @dydxprotocol/security.
  • Impacts GitHub’s reviewer auto-assignment and, if enabled in branch protection, required code owner reviews.

Risk & Impact

• Low risk: repository metadata only; no runtime or build impact.
• Workflow change: PRs touching .github/ and CODEOWNERS will route to/require security team review (subject to branch protection settings).

Testing

• No tests applicable; behavior validated via GitHub’s CODEOWNERS mechanics.
• Expectation: opening PRs should auto-request the specified owners.

Reviewer Notes

• Verify branch protection requires code owner reviews if the intent is to enforce security sign-off on policy/config changes.
• Confirm the owning teams exist and have appropriate permissions.