Add example consumers and sinks

[fschoell]

• [ ] add a consumer example to retrieve data from the blocktime-meta substream
• [ ] add an example sink for the above substream
• [ ] add documentation on how to write custom consumers
• [ ] add documentation how to use the existing sinks
  • [ ] flat file
  • [ ] MongoDB
  • [ ] graph-node

[DenisCarriere]

This would be documented via StreamingFast docs or Graph docs