store: decide on encoding

[liamsi]

Decide which encoding to use when serializing into the local data store.

ref: #42 (comment)

[tzdybal]

This encoding is important also for DA layer submission.

[tzdybal]

Some options for efficient serialization:

• protobuf - well known and widely used in cosmos ecosystem
• Apache Avro

[tzdybal]

This issue was resolved by #57 - we decided to use protobuf.