rpc: Export Client trait unconditionally

[artemii235]

Description

As of now, Client trait is defined and exported only if http-client or websocket-client features are enabled:
https://github.com/informalsystems/tendermint-rs/blob/main/rpc/src/lib.rs#L39

So, if crate users prefer to implement their own client, they have to enable features, which they don't need. Enabling these also leads to adding a bunch of unneeded subdeps.
In our project, we had to copy the entire trait definition to work this around:
https://github.com/KomodoPlatform/atomicDEX-API/pull/1546/files#diff-f0f8ba08e587c05523df40e172e6073ab961cdd4517eda6073b14f17d07e1d66R30

Definition of "done"

Define and export Client trait, not depending on http-client and websocket-client features.