chore: Update CODEOWNERS to identity (#130)

## Summary

This pull request updates the CODEOWNER from MetaMask/Engineering to
MetaMask/Identity. This is done as part of efforts to reduce the noise
caused by having large teams as codeowners, and better attributing
ownership to repositories.

The owner property of this repository has also been updated accordingly
here: https://github.com/MetaMask/message-signing-snap/custom-properties

Author: NicholasEllul

.github/CODEOWNERS

@@ -1,4 +1,4 @@
 # Lines starting with '#' are comments.
 # Each line is a file pattern followed by one or more owners.
 
-*                                    @MetaMask/engineering
+*                                    @MetaMask/identity