Add integration method to sensor.integration

[dgomes]

Description:

There are various numerical integration methods, and although trapezoidal is the most common, cases are where left or right method are more appropriate.
This PR adds an extra configuration option to make the left and right riemann methods available in addition to the default trapezoidal method.

Related issue (if applicable): fixes https://community.home-assistant.io/t/different-modes-for-integration-sensor/98827

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#8557

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: integration
    name: integration
    method: left
    source: sensor.power
    unit: kWh
    round: 2

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.

[OttoWinter]

LGTM 👍