Overall changes:
- [x] introduce `Resp2Type` and `Resp3Type` shims (`Resp2Type` has reduced types); existing code using `[Obsolete] Type` uses `Resp2Type` for minimal code impact
- [x] mark existing `Type` as `[Obsolete]`, and proxy to `Resp2Type` for compat
- [x] deal with null handling differences
- [x] deal with `Boolean`, which works very differently (`t`/`f` instead of `1`/`0`)
- [x] deal with `[+|-]{inf|nan}` when parsing doubles (explicitly called out in the RESP3 spec)
- [x] parse new tokens
- [x] `HELLO` handshake
  - [x] core message and result handling
  - [x] validation and fallback
- [x] prove all return types (see: redis/redis-specifications#15)
- [x] <strike>streamed RESP3</strike> omitting; not implemented by server; can revisit
- [x] deal with pub/sub differences
  - [x] check routing of publish etc
  - [x] check re-wire of subscription if failed
  - [x] check receive notifications
  - [x] connection management (i.e. not to spin up in resp3)
  - [x] connection fallback spinup (i.e. if we were trying resp3 but failed)
- [x] other
  - [x] [undocumented RESP3 delta](redis/redis-doc#2513)
  - [x] run core tests in both RESP2 and RESP3
  - [x] compensate for tests that expect separate subscription connections

Co-authored-by: Nick Craver <nickcraver@microsoft.com>
Co-authored-by: Nick Craver <nrcraver@gmail.com>