Implement builder constructs to build the relay contexts

[soareschen]

Summary

Currently, we have built abstraction around implementing the relay contexts, and provide concrete types such as CosmosFullRelayContext for running the relayer. However, the construction of such types is pretty complicated, as it involves calling many constructors such as OfaRelayWrapper::new(CosmosRelayWrapper::new(...)) to construct the relay context. Furthermore, there are special care need to be taken such as cloning the chain contexts for each relay context, and spawning the batch message workers.

To make it easier for users to instantiate a concrete relayer, we will make use of the same context-generic programming pattern to define builder constructs for building the relayer. At the high level, external users only need to define a builder context and implement traits like OfaBuilder to construct the relayer.

Details

The builder constructs abstract away the complexity of building programs, i.e. implementing build systems. Traditionally, we know that build systems like Make, Ninja, Bazel are complex because they need to keep track of build dependencies and create a suitable plan for building the programs. Using context-generic programming, we can manage some of those complexities within Rust itself, by defining builder traits that depends on other builder traits declaratively.

The builder constructs will help make sure that build dependencies can be constructed declaratively and reused. For instance, a chain context is built once and reused by all relay contexts, and each relay context is built once with one batch message worker running.