[Rust Server] Pass context to client middleware

[richardwhiuk]

When creating a client with middleware, pass the context.

This allows users to pass contextual data about the requests through to the
client middleware.

Note, this is breaking, because:

a) middleware now needs to pass in a DropContextService wrapper when calling Client::try_new_with_client_service() if it's not expecting to use the context.

b) the type of a client has changed (as the Service is now wrapped in a DropContextService).

Rust Server Technical Committee

• @frol
• @farcaller
• @bjgill
• @paladinzh

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
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