Minimize prost dependency via ToVec trait

[Farhad-Shabani]

Summary

The prost should ideally be only a dependency of the *-types crates, as it essentially facilitates back and forth between domain and proto types.

Proposal

We can introduce a ToVec trait and blanket implement it for types that implement prost::Message. This modification would enable the removal of the prost from all the module-level implementations.

[Farhad-Shabani]

Further investigation revealed we even can move away from prost in all the *-types crates and manage prost only in the ibc-pritmitves. This improvement decouples our errors from prost and paves the way for refactoring our error-handling system through this issue: #270