Add P-Chain ctx into inner block and verify within snow pkg

[aaronbuchwald]

This PR adds the P-Chain context into the inner block.

This embeds it directly into the inner block, so that it's always available to the inner block. By adding this into the inner block interface, we can verify it against the consensus context in the snow package, so that the inner VM/chain does not need to.

[aaronbuchwald]

b4ec9ce fixes a bug where the block implements block.WithVerifyContext, but the VM did not implement block.BuildBlockWithContextChainVM so we'd build a block without the context and verify it with the P-Chain context.

Needed to add the covariant version of BuildBlockWithContext to implement the interface correctly and fixes the issue.

[aaronbuchwald]

ref on ProposerVM: https://github.com/ava-labs/avalanchego/tree/master/vms/proposervm#snowman-congestion-control-for-snowman-vms

We need the ProposerVM block context to verify chunk signatures for DSMR and for ICM signature verification.

[RodrigoVillar]

Should we change the struct tag from json:"pChainHeight" to json:"blockContext"?

[tsachiherman]

same thought here. The inner BlockContext object would already have a json field name PChainHeight.

[aaronbuchwald]

Ya good callout

[aaronbuchwald]

It only contains P-Chain height, but this would be better