create a client feature in kube #525

[clux]

Makes pretty much every heavy dependency optional (which does make things a bit confusing), but it's now possible to build kube without default-features and you'll get something quite similar to kube-core, something that is small enough that kube-derive can depend on it.

I think in theory we can actually make every dep here optional because without default features, all we do is re-export kube-core, but I think the ones i left are in kube-core anyway...

Tried a bit to make a config and client feature (possible in theory to build config without client, but not the other way around), but there are many dependencies in the shared set atm so it would include some more complicated cfg! selectors. Maybe a future issue.

Replaces #526 (that fix is included) and #527 (i think).

[clux]

cc @kflansburg

[kazk]

Suggested change

	default = ["native-tls", "client"]
	native-tls = ["openssl", "hyper-tls", "tokio-native-tls"]
	rustls-tls = ["hyper-rustls", "tokio-rustls"]
	default = ["native-tls"]
	native-tls = ["client", "openssl", "hyper-tls", "tokio-native-tls"]
	rustls-tls = ["client", "hyper-rustls", "tokio-rustls"]

I think this works.
We can also add client to other features that doesn't make sense without client like ws, oauth, gzip.

[clux]

this does introduce having features depend on features also, which will make them slightly harder to follow if it gets heavily used. it probably isn't a problem either way, put a static assert for this case in lib.

[clux]

Thinking more about it, this does seem like a better way to declare feature dependencies than with assertions. Will continue tomorrow

[kazk]

Yeah, features should be additive, but some features depends on another implicitly already. I think it's better to make that explicit.
Not sure how common, but I've seen this pattern used in some crates I've used.

[clux]

Have added client to most client-using feature sets and removed the static assert.

Also separated the dependencies into client, __config and __non_core in the hopes that we can make a proper config feature.

[kflansburg]

FYI, I was able to build a WASM target using this branch with default-features = false. 👍

[kazk]

Maybe treat client feature internal? Looks good to me other than that.

[clux]

Ah, yeah, should keep in spirit with this now being an unbreaking change. Did all your suggestions in one commit.

[clux]

Thanks for the very detailed review! I'll merge this now, release it later / tomorrow.