Allow to call arbitrary runtime apis using RelayChainInterface (#5521)

When using the relay chain though a `Arc<dyn RelayChainInterface>` there
is no way to call arbitrary runtime apis. Both implementations of that
trait allow this, so it feels natural to expose this functionality in
the trait.

This PR adds a `call_runtime_api` method to RelayChainInterface trait,
and a separate function also named `call_runtime_api` which allows the
caller to specify the input and output types, as opposed to having to
encode them. This generic function cannot be part of the trait because a
`dyn Trait` object cannot have generic methods.

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Co-authored-by: Bastian Köcher <[email protected]>