Implement minimal support for backward verification in light client

[romac]

The IBC relayer may occasionally encounter headers to verify which are below the latest trusted state of the light client for the chain.

To support this use case, we need to add support for backward verification to the light client.

I suggest we initially start with a minimal implementation which requires the fewest changes to the light client, at the expense of performance. This first implementation will be guarded behind an opt-in unstable feature flag to signal its early state.

See part V of the English specification for more information: "Supporting the IBC relayer"