Add CODEOWNERS file for team ownership structure (#300)

### Summary

Add CODEOWNERS file.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

## Release Notes

* **Chores**
* Updated repository configuration for internal team ownership and code
review assignment. No end-user-facing changes.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: martyngigg

.github/CODEOWNERS

@@ -0,0 +1,11 @@
+# default ownership falls to the whole team
+* @ISISNeutronMuon/analytics-team
+
+# infrastructure
+/infra/ @ISISNeutronMuon/analytics-team-code-owners-infra
+/certs/ @ISISNeutronMuon/analytics-team-code-owners-infra
+/.github/ @ISISNeutronMuon/analytics-team-code-owners-infra
+
+# pipeline code
+/elt-common/ @ISISNeutronMuon/analytics-team-code-owners-pipelines
+/warehouses/ @ISISNeutronMuon/analytics-team-code-owners-pipelines