Add option to ignore availability in google calendar events

[cgtobi]

Description:

This implements the transparency feature for google calendars. If for example an all-day event is present but it is set to 'free' HA now skips those events and shows the next. This can be turned on/off per calendar.

ignore_availablilty is optional and defaults to false. It enables/disables whether to respect/ignore the transparency tag which can be opaque (busy) or transparent (free).

Related issue (if applicable): fixes #6076 (only partial)

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#5101

Example entry for configuration.yaml (if applicable):

- cal_id: calendar@googlemail.com
  entities:
  - device_id: calendar
    name: Calendar
    track: true
    ignore_availablilty: true

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.

If the code does not interact with devices:

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

[syssi]

Ping me if the config is fixed.

[syssi]

Looks good! Thanks for the code clean-up.

[syssi]

Could you check/update the tests? One is failing.

[cgtobi]

Yeah, I just fixed that. Should be working fine now.