Allow pluggable BEEFY payload constructors

[acatangiu]

Now with #198 implemented it should also be possible to customize the payload construction in the gadget code instead of having it hardcoded.

[acatangiu]

The BEEFY worker (located here) could vote on generic payloads that can hold anything encodable as [u8].

This issue tracks enabling the worker/voter to use pluggable payload constructor. So for example BeefyWorker::do_vote() will no longer explicitly encode MMR_ROOT in the vote payload, but will use the externally provided payload constructor, thus being agnostic to the actual payload contents.

The "pluggable payload constructor" can, for example, be a trait object that builds Payload given a block header.

For example, a custom/pluggable runtime api provider object was added to beefy-gadget by this commit. Something similar is required in this issue.

A simple test for this would be to verify generated SignedCommitments (see for example this function in tests).

[acatangiu]

Fixed in paritytech/substrate#12428