Create CODEOWNERS (#1129)

## What was changed
Add CODEOWNERS so that Security can own the Semgrep rules files and
paths.

## Why?
We are adding Semgrep for static analysis to this repository, and only
the security team should be able to approve exclusions from the policy.

## Checklist

How was this tested:
We ran this scanner on internal repos with this CODEOWNERS file and it
worked as expected.

Author: jackdawm

.github/CODEOWNERS

@@ -0,0 +1,11 @@
+# These owners will be the default owners for everything in
+# the repo. Unless a later match takes precedence,
+# @temporalio/sdk will be requested for review when
+# someone opens a pull request.
+* @temporalio/sdk
+
+# Security team must approve exceptions to static analysis
+# scans, and any custom rules.
+
+.semgrepignore @temporalio/security
+.semgrep/ @temporalio/security