Add a new light client type using a module

[majecty]

We can implement light clients of ICS in two ways:

1. Implement light client logic in the Foundry Rust source code.
2. Implement light client logic in a module

If a programmer wants to use ICS with another chain whose light client is not implemented in Foundry yet, they will be happy if the light client can be implemented using the module system.

However, implementing a correct light client is hard work. A simple error can violate security. It would be better not to allow the module programmer to create a light client if we consider chain security.

[junha1]

I think security importance is same for any other modules. so I believe it's ok to allow that

[majecty]

We concluded to implement a new light client type using the module system. Then we don't need to consider the light client and Merkle proof abstraction for other chains. To support multiple light client types, we will implement light client modules for each chain.

[junha1]

Note this is completely different from the stand alone light client (not for the ICS) as a module in future.

[majecty]

This is not an "experiment" issue. The topic of this issue is how to implement ICS light client in the final version (not PoC version).

[majecty]

Since we concluded to implement light client using a module. I'll close this issue.
Light client module's interface is described in here: #174