transport/grpc: support option.WithAPIKey

google.golang.org/ap/option says:

```
API Keys can only be used for JSON-over-HTTP APIs, including those under the import path google.golang.org/api/....
```

Not true, though, API keys can be used with gRPC.

Need to use the x-api-key metadata key:

```
ctx = metadata.NewOutgoingContext(ctx, metadata.Pairs("x-api-key", ...))
```