Refactor the ChainEndpoint trait to expose the light client functionality directly

[romac]

Summary

Refactor the ChainEndpoint trait to expose the light client functionality directly instead of via a getter for a LightClient.

Problem Definition

The chain runtime is too coupled to the light client (the former should ideally not need to know about the latter). By decoupling the two we could simplify the runtime initialization code and keep a cleaner SoC.

Proposal

Instead of exposing a getter for the LightClient trait, the ChainEndpoint trait should now define the two methods verify_header and check_misbehaviour directly.

Acceptance Criteria

As above.

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