Split out iperf3 into a component with a sensor platform

[rohankapoorcom]

Description:

Per home-assistant/architecture#131, sensor platforms should not be registering services. iperf3 is one of several that does and this should be cleaned up. This sensor has a service call which is used to manually run a speedtest and then updates the sensor with that information.

I moved all of the setup/service logic to a new iperf3 component and then used that component
inside the sensor platform. All configuration has been moved to the component and the sensor platform is loaded automatically when the component is loaded.

Using a dispatcher signal, the component notifies the platform when it's finished a speedtest so the platform can update it's data.

Same as #20341, #20527 but for iperf3.

Breaking Change: The iperf3 sensor platform has been separated into a component and a sensor to remove the service from the sensor platform. Configuration options have changed, please review the documentation and select the correct options.

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#8750

Example entry for configuration.yaml (if applicable):

iperf3:
  hosts:
    - host: iperf.he.net

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

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

• Documentation added/updated in home-assistant.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 or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[MartinHjelmare]

Can be merged when build passes.