Extend client context mock to support more realistic unit testing

[adizere]

Summary of Bug

Sub-issue of #115.

Currently, the ICS02 mock context supports only 1 client and 1 consensus state for that client:
https://github.com/informalsystems/ibc-rs/blob/cf6c815eb03ddbee811c5220935f8b96c6b48440/modules/src/ics02_client/context_mock.rs#L11-L16

Consequently, this mock permits very limited testing scenarios.

Rough roadmap:

• extend struct MockClientContext to permit storing multiple consensus_state,
• extend the same struct to support multiple client_state (indexed by their client_id most likely)
• add additional testing scenarios to both in ICS02 (create & update client) and ICS03 (init & try handlers)

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