[WIP] Add support for gRPC binary metadata values

[jazev-stripe]

What was changed

(this description is a work-in-progress)

This PR depends on the changes in temporalio/sdk-core#997 (and temporarily updates the submodule to reference that PR's changes).

Why?

#1063

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

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