Add camera selection config to skybell camera

[nhorvath]

Description:

Ongoing conversation in #17617 resulted in a better fix. Some people liked the image we used when the original broke, so I am adding a config to let you choose which image to use in Skybell Camera.

Related issue (if applicable): fixes #17617

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#7849

Example entry for configuration.yaml (if applicable):

camera:
  - platform: skybell
    monitored_conditions:
    - avatar
    - activity
    activity_name: "Last Activity"
    avatar_name: "Current"

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.

[nhorvath]

@MartinHjelmare this is a follow up to #19100 (review). I was able to fix the underlying issue, but added some configs to the camera so people could keep the current functionality if they wanted.

I'm pretty sure the failed test is unrelated to my change. I'm not sure how to make it rerun to see if it's just a mistake.

[MartinHjelmare]

Looks good! Can be merged when build passes.