Metadata Api should be optional

[JoshOrndorff]

Currently the Substrate client requires that every runtime implement the Metadata runtime Api. But not all runtimes will need this Api. When writing a simple runtime, probably without frame, you have to stub this runtime api with something like this

impl sp_api::Metadata<Block> for Runtime {
	fn metadata() -> OpaqueMetadata {
		OpaqueMetadata::new(vec![0])
	}
}

Is metadata intended for use in FRAME only? If so, I don't think it should be required.

cc @kianenigma

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1. Remove sp-api requirement on compile time bounds polkadot-sdk#27
   I4-refactor I7-refactor +2
   

[kianenigma]

cc @bkchr @koute

[bkchr]

I think it is probably because of the Metadata rpc? The client itself doesn't need the Metadata. In the future this will be fixed™

[kianenigma]

As far as I understood, similar to #14198, all such bounds would go away after paritytech/polkadot-sdk#27. If confirmed, let's close this one as well in favor of it.