Add network throughput statistics to systemmonitor sensor

[dgomes]

Description:

The system monitor sensor provides counters (bytes and packets) but does not provide throughput.

Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#<home-assistant.io PR number goes here>

Example entry for configuration.yaml (if applicable):

- platform: systemmonitor
  resources:
    - type: throughput_network_in
      arg: ens3
    - type: throughput_network_out
      arg: ens3

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 does not interact with devices:

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