Add bridge dispatcher and exporter

[svyatonik]

The fully working bridge may be started by using branches, mentioned in paritytech/parity-bridges-common#2547. I'm going to open multiple PRs to make it easier to review - some PRs need a different set of reviewers

This PR adds message dispatcher for inbound messages and exporter for outbound messages. Two differences with our "regular" configuration:

• inbound messages are not pushed onto some queue (because there are no any queues in runtime), but instead are dispatched immediately;
• because of that ^^^, there's a need for computing dispatch weight using XcmExecutor, hence the WithXcmWeightDispatcher.

@acatangiu @bkontur @serban300 Please, take a look.

[acatangiu]

LGTM!

[bkontur]

lgtm,

I am just curious, maybe I've missed that somewhere,
I see type MessageExporter = ToBridgeHubPolkadotHaulBlobExporter;,
so just one question, what kind of calls will send this chain to the BridgeHub?

[svyatonik]

lgtm,

I am just curious, maybe I've missed that somewhere, I see type MessageExporter = ToBridgeHubPolkadotHaulBlobExporter;, so just one question, what kind of calls will send this chain to the BridgeHub?

I have been said that they'll be calling some stuff on that new Polkadot parachain (referenced as Kawabunga here). So probably also Transact without any fees. For now I've just added an option to send messages and then we may need to fix something at Polkadot.BH (barriers?) when we'll have a messages and it'll be close to deployment. Right now I only care about adding a transport - when the app on top of that transport will be ready, we'll see.

[svyatonik]

@zdave-parity When you have a moment, please review and approve if it is ok - we need an approval to merge. Thanks