[tracking] Release 0.20.0

[jsdw]

A rough plan to work towards for the next release.

• Static metadata validation #398
• Simplify Extrinsic Creation and Signing #477 (which should also close Remove pub from various fields in exported types, notably in extrinsic::extra #390 and Prioritize a transaction by tip #187)
• Put integration tests behind feature flag to avoid need for substrate on PATH #351 (Minor)
• Use tokio instead of async-std. #485 (If we agree it's worth doing, it should be minor)

Future work/Nice to Haves

• Feature request: subscribing to storage changes #479 (Let's come up with a nice API for this)
• Generate pallet RPC calls #416 (Would be nice to explore this and figure out what needs doing!)
• Add more dynamic decoding support to subxt #406 (Would be nice to work up a rough design for this)
• Dynamically derive default mortality from System::Constants::BlockHashCount #206
• Feature request: subscribing to storage changes #479 (Something to give some thought to)
• Not able to clone Api Client #350 (Should/can we make it easier to clone and share api clients around to avoid creating new ones with their own copies of metadata etc?)