feat: add --extra-header flag for custom gRPC headers

[sputnikraket]

Signed-off-by: Erik Hennings erik.hennings@freda.com

Description

This PR adds support for passing custom headers to gRPC requests via a new --extra-header flag.

Testing

Manual testing

References

[github-actions]

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[sputnikraket]

I have read the CLA Document and I hereby sign the CLA

[tstirrat15]

I think this is fine, but what's the use case? I'm wondering if it's something that zed already supports.

[radhus]

I think this is fine, but what's the use case? I'm wondering if it's something that zed already supports.

We're running SpiceDB behind a proxy that requires authentication (GCP Cloud Run), that can be specified either through the Authorization header or another header. So we kind-of need to pass double authentication tokens 😅

This setup is perhaps not ideal, but it works.

[tstirrat15]

See question; otherwise LGTM

[tstirrat15]

Does this logic functionally merge the headers, or would this overwrite headers if some other component added new headers?