Extract client-traits crate

[kevinheavey]

Problem

solana_sdk::client imposes a solana_sdk dep on solana_rpc_client

Summary of Changes

Move to its own crate and re-export with deprecation

This branches off #3634 so that needs to be merged first

[mergify]

If this PR represents a change to the public RPC API:

1. Make sure it includes a complementary update to rpc-client/ (example)
2. Open a follow-up PR to update the JavaScript client @solana/web3.js (example)

Thank you for keeping the RPC clients in sync with the server API @kevinheavey.