Scaffold the Sovereign chain and relay contexts

[soareschen]

Part of: #10

Summary

This will lay the basic skeletons for collaboration between multiple developers to implement the Sovereign relaying.

• Create the initial crate boilerplates such as hermes-sovereign-components and hermes-sovereign-relayer.
• Define minimal concrete context structs such as SovereignChain, SovereignToCosmosRelay, CosmosToSovereignRelay with some placeholder implementations to be filled in by subsequent PRs.
• Write some placeholder tests for testing the Sovereign implementations.

Note that the actual testing of the Sovereign implementation can only be done after #94 is completed. However in the meanwhile, we can define the scaffolding to allow work to be done in parallel.