Support forwarding binary headers

[mchang-stripe]

Description & motivation 💭

This PR adds support for forwarding binary gRPC headers (headers ending with -bin suffix) from HTTP requests to gRPC metadata.

Motivation:
Binary headers in gRPC are base64-encoded when transmitted over the wire, but must be stored as plain text in gRPC metadata. Previously, the header forwarding middleware would forward binary headers as-is (base64-encoded), which would cause double-encoding issues when the gRPC library re-encodes them for transmission.

Changes:

• Modified WithForwardHeaders to detect binary headers (those ending with -bin)
• Binary headers are now base64-decoded before being added to gRPC metadata
• Regular headers continue to be forwarded as-is
• Supports both standard base64 (with padding) and raw base64 (without padding)

Screenshots (if applicable) 📸

Design Considerations 🎨

Testing 🧪

How was this tested 👻

• Manual testing
• E2E tests added
• Unit tests added

Steps for others to test: 🚶🏽‍♂️🚶🏽‍♀️

Checklists

Draft Checklist

Merge Checklist

Issue(s) closed

Docs

Any docs updates needed?

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