Offchain Worker Api should be optional

[JoshOrndorff]

Currently the Substrate client requires that every runtime implement the Offchain worker runtime Api. Many (maybe even most?) runtimes do not use offchain workers and have to stub this Api like

impl sp_offchain::OffchainWorkerApi<Block> for Runtime {
	fn offchain_worker(_: &Header) {}
}

While it isn't the hardest thing to paste that code into your runtime, it adds an entire dependency on sp_offchain for runtimes that don't otherwise need it.

I suggest the offchain worker api not be required.

cc @kianenigma

[kianenigma]

Probably has the same answer as #11743, so will close to keep our enormous issue backlog at least one less :D