Add CODEOWNERS file

[jmagman]

#509 caused App Store submission failures for Flutter customers and several g3 apps.

Create a CODEOWNERS file. Add myself to build/config/ios so I am automatically added to any reviews of this file so I get a heads up when these settings are changed. The codeowner is not a blocking reviewer, but is automatically added to any PRs that change the file.

$ git ls-files build/config/ios/
build/config/ios/BUILD.gn
build/config/ios/ios_sdk.gni
build/config/ios/ios_sdk.py

See also https://github.com/flutter/flutter/blob/master/CODEOWNERS.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[jmagman]

@zanderso how do I handle The base branch requires all commits to be signed. for a process PR like this one?

[zanderso]

@Hixie may know.

[jmagman]

Let me just sign the commit.

[Hixie]

I can disable that if it's trouble, I enabled it for a bunch of repos as part of an audit but we may need to roll that particular one out with more documentation on how to actually sign commits.

[jmagman]

I can disable that if it's trouble, I enabled it for a bunch of repos as part of an audit but we may need to roll that particular one out with more documentation on how to actually sign commits.

It took about 30 minutes but I figured it out on macOS...