making InjectedCheckpoint a standard tx

[SebastianElvis]

Summary

It would be good to make InjectedCheckpoint to implement standard tx interfaces, such that any client (eg indexer) can parse it.

Problem Definition

InjectedCheckpoint is a special tx inserted to the 1st block of each epoch (ref). It represents as the checkpoint of the previous epoch, certified by >2/3 validators. However, it represents as a protobuf object and does not implement tx interfaces, such that clients cannot unmarshal it following Cosmos SDK tx format.

Proposal

It would be good to make InjectedCheckpoint to implement standard tx interfaces, such that any client (eg indexer) can parse it.