envoyconfig: add authority header to outbound gRPC requests

[calebdoxsey]

Summary

Update the envoy config for outbound gRPC requests so that the authority header gets rewritten to match the cluster's destination address.

Related issues

Fixes https://github.com/pomerium/pomerium/issues/3505

User Explanation

Fixes an issue with route matching for Istio.

Checklist

• [x] reference any related issues
• [ ] updated docs
• [x] updated unit tests
• [ ] updated UPGRADING.md
• [x] add appropriate tag (improvement / bug / etc)
• [ ] ready for review

[coveralls]

Coverage increased (+0.2%) to 65.497% when pulling 23d496e9e4b394ef7bc1f42e32d3457ee808495d on cdoxsey/outbound-authority into f87e138eab2a0bc6f3a58c6f1018a5d930585ff8 on main.

[desimone]

Marking as blocked until we can confirm the fix from @bennycooly