update codeowners file and make apm serverless co-owners of repo (#596)

Author: happynancee

.github/CODEOWNERS

@@ -1,6 +1 @@
-*                               @DataDog/serverless-aws
-datadog_lambda/tracing.py       @DataDog/apm-serverless
-datadog_lambda/patch.py         @DataDog/apm-serverless
-datadog_lambda/span_points.py   @DataDog/apm-serverless
-datadog_lambda/cold_start.py    @DataDog/apm-serverless
-datadog_lambda/wrapper.py       @DataDog/apm-serverless
+*                               @DataDog/serverless-aws @DataDog/apm-serverless