Implement GRPC Streaming config in Xatu output

[samcm]

Note: this might be too painful when combined with the batch processor, but it's worth investigating.

The Xatu output sends individual RPC requests to xatu-server.EventIngester. We should support streaming via this output, swappable by config:

https://github.com/ethpandaops/xatu/blob/master/pkg/output/xatu/config.go

This config should also support setting TTL on the stream, and any other stream-oriented config.

xatu-server.EventIngester will need to be updated to support streaming.