[DRAFT] Extend 02-client w/ Conditional Clients

[notbdu]

Modular blockchains break up a logical blockchain into many constituent parts. To accurately represent these chains and also to account for various types of shared security primitives that are coming up, we need to introduce dependencies between clients.

This is a DRAFT spec change. Opening for discussion.

[AdityaSripal]

Happy to see the changes are clean and minimal!