Converts RainMachine to hub model (part 2)

[bachya]

Description:

#14085 was the first cut at moving RainMachine to a hub/component model (for future functionality). After discussion with @balloob, I want to tweak things so that the rainmachine component forwards discovery of entities to the platforms (which will make configuration easier, as shown below).

BREAKING CHANGE: #14085 (already merged) broke the existing format; this will do so again. If we can merge this before #14085 is released, we will limit to one breaking change (ideal) and can use the message described in that PR.

CC: @MartinHjelmare

Related issue (if applicable): N/A

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#5251 (although this related to #14085 originally, since it hasn't been merged, going to hustle and use it for this PR, as well)

Example entry for configuration.yaml (if applicable):

# Bare Configuration:
rainmachine:
  ip_address: 192.168.1.100
  password: abc123

# Configuration of Existing Switch Parameter:
rainmachine:
  ip_address: 192.168.1.100
  password: abc123
  switches:
    zone_run_time: 240

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:
- [ ] New dependencies have been added to the REQUIREMENTS variable (example).
- [ ] New dependencies are only imported inside functions that use them (example).
- [ ] New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
- [ ] New files were added to .coveragerc.