Add a config option to enable gRPC SharedWriteBuffer server option

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Add a config option to configure gRPC to use SharedWriteBuffers.

By itself this change does nothing. The gitaly toml will be used to roll this change out.

Why isn't this a feature flag? This option is applied to the gRPC server when it is created. Feature flags are request scoped, and as such cannot be used at the gitaly startup time. So a config option seems like the best option here.

Closes #6325