Add draft CODEOWNERS file.

[jimmielin]

Originator(s): @jimmielin

Description (include issue title and the keyword ['closes', 'fixes', 'resolves'] and issue number):

• Following up from the 03/04/2025 CAM-SIMA planning meeting I drafted a CODEOWNERS file for the purpose of facilitating code review requests and/or "who do I ask to learn about this scheme?" in the future!
• Closes Create CODEOWNERS file #216

List all namelist files that were added or changed: N/A

List all files eliminated and why: N/A

List all files added and what they do:

CODEOWNERS codeowners file.

List all existing files that have been modified, and describe the changes:
(Helpful git command: git diff --name-status development...<your_branch_name>)
N/A

List all automated tests that failed, as well as an explanation for why they weren't fixed:
N/A

Is this an answer-changing PR? If so, is it a new physics package, algorithm change, tuning change, etc?
N/A - codeowners file only

If yes to the above question, describe how this code was validated with the new/modified features: