imp(ibc-testkit): refactor HostBlock with a trait

[rnbguy]

Details

Currently, HostBlock is implemented as an enum of different host variants.

ibc-rs/ibc-testkit/src/hosts/block.rs

Lines 72 to 75 in d5e8314

	pub enum HostBlock {
	Mock(Box<MockHeader>),
	SyntheticTendermint(Box<SyntheticTmBlock>),
	}

This made its implementation a manual dispatching of methods from MockHeader and SyntheticTmBlock. We can avoid this by implementing a HostBlock trait for these two and generalizing an IBC context with a generic with the HostBlock trait bound.

Also, this allows a downstream project to reuse our MockContext with their host block variant.

This was blocked before because the ClientState derive didn't support generics. Since #1016 is resolved, we can use generics with trait bounds in an IBC context.

Version

<= v0.50.0

[rnbguy]

This is an improvement over HostType removal as part of #677

[rnbguy]

closed by #1109