Feature gate client side of parachain inherent

[JoshOrndorff]

This PR adds a client-side feature to the cumulus-primitives-parachain-inherent crate. Previously the client-side features were gated only with the std feature. This solution was not ideal because runtimes are also built to std, and when they are the entire client-side of this crate is built including its dependency on polkadot runtime which adds a lot of compile time.

With this new feature, we can express whether we want the client side or not without abusing the std feature.

[bkchr]

This will not solve any problem that you have. When this feature is enabled somewhere in your workspace, it will always be enabled.

There will be coming changes in the near future (hopefully until the end of the year) which will remove the dependency on the polkadot-runtime in this case.

[JoshOrndorff]

Can you share your idea for coming changes? Is there an issue with a description?

I was thinking last night that a primitives crate is supposed to be useable in the runtime and client side, whereas a client-side crate (think sc- crates in Substrate should contain the client side stuff. So it doesn't make that much sense for the client side stuff to live in the crate cumulus-primitives-parachain-inherent to have the client side stuff anyway.

[bkchr]

#545

[bkchr]

That is the issue that will fix your complain :P