omni-node: add metadata checks for runtime/parachain compatibility

[iulianbarbu]

Description

Get runtime's metadata, parse it and verify pallets list for a pallet named ParachainSystem (for now). Ideally we'll add others pallets checks too, at least a small set of pallets we think right away as mandatory for parachain compatibility.
Closes: #5565

Integration

Runtime devs must be made aware that to be fully compatible with Omni Node, certain naming conventions should be respected when defining pallets (e.g we verify parachain-system pallet existence by searching for a pallet with name ParachainSystem in runtime's metadata). Not finding such a pallet will not influence the functionality yet, but by doing these checks we could provide useful feedback for runtimes that are clearly not implementing what's required for full parachain compatibility with Omni Node.

Review Notes

• I am unsure about how "generic" this metadata inspection logic should be. This might seem "overengineering", but I think we might add similar "pallet" checks in the future, to assert that a runtime has full parachain compatibility (TBD what this is and which are the pallets we want to check against). The checking logic depends only on the decoded runtime's metadata (frame_metadata::RuntimeMetadataPrefixed), obtained from an opaque bytes array. The logic could reside in frame_metadata, attached to RuntimeMetadataPrefixed type. I'll consider better ways of encapsulating the logic after clarifying the other metadata checks we could add in this PR.
• TODO: check frame_system's metadata to ensure the block number in there is the same as the one in the node side
• TODO: update omni node polkadot-sdk docs to make these conventions visible (maybe worth mentioning them somewhere else? I have to check)
• TODO: handle unstable metadata (v16) too
• TODO: add tests for the checking logic