HeaderChain trait

[tomusdrw]

Define & implement the HeaderChain interface to cover current and future bridge pallets. Implement HeaderChain for PoA bridge pallet. Proposed interface:

fn import_header_unsigned(Self::Header, Self::Extra);
fn import_headers_signed((Self::Header, Self::Extra)[]);
fn best_block() -> Self::Header;
fn finalized_block() -> Self::Header;
fn earliest_block() -> Self::Header; // earliest useful for proofs
fn by_number(Self::BlockNumber) -> Self::Header;
fn by_hash(Self::BlockHash) -> Self::Header;
fn verify_proof(Self::Proof) -> bool;
// I imagine Proof to be an enum in case multiple things can be proven
// (Transaction, Event, MultipleBlocks, ValidatorSet, etc)

Triggered hooks:

fn on_valid_headers(
	Option<Submitter>, useful: Self::Header[], useless: Self::Header[]
);

fn on_invalid_headers(Option<Submitter>);

Proof for each implementation will most likely be an enum containing different things that can be proven (think Transaction or Event, etc). For that reason it will probably be required to have traits that allow proof creation, like so:

trait TransactionProof<T> { type ProofData; fn new(T::BlockHash, Self::ProofData) -> Self }

trait EventProof<T> { type ProofData; fn new(T::BlockHash, Self::ProofData) -> Self }

So that for instance CurrencyExchange (see #205) can specify someting like this:

/// Blockchain is a trait that is required by CurrencyExchange currently.
impl<T: HeaderChain> Blockchain for T where
 T::Proof: TransactionProof<T>,
{
...
}

```

[tomusdrw]

Closed via #287