polkadot-omni-node: Metadata checks

[kianenigma]

As we move towards making polkadot-parachain be a true omni-node, independent of the runtime, we should make some checks.

For example, for now we only support a parachain, and therefore parachain_system pallet should be present.

Some startup checks should exist that query the runtime for the metadata, parse it using frame-metadata and/or subxt, and assert the existence of some types.

Further, now we allow the block number type to be configured on the omni-node. One can check frame_system's metadata to ensure the block number in there is the same as the one in the node side.