Feature configurable scan interval (#29)

[LavermanJJ]

This PR fixes #29 by making the scan-interval configurable:

• Add scan-interval as data to be entered when adding the integration (default remains 60s, minimum 5s)
• Add config-flow migration, updating existing configurations by adding new scan-interval data
• Add option-flow, allowing to update the scan-interval w/o re-installing the integration

Notes to @mchwalisz :

• I've developed code within the home-assistant dev-container, using it's default formatting. So please excuse the formatting changes 🙈
• I've removed some unused imports. Hope the un-related cleanup is okay

Looking forward for your feedback!

[LavermanJJ]

New scan-interval option, when installing integration:

Integration offering option flow "configuration":

Options screen:

[lennartschulte]

I don't see the "Configure" Option in the new HA integration overview.

[mdopp]

For me i can say, that I can see the config-parameter, and that I am able to enter i.E. "25"

But the build fails due to an unsorted manifest.json (domain, name, rest alphabetical). This has nothing to do with this pull request, but breaks the checks currently. Maybe @LavermanJJ you could also sort this?

[LavermanJJ]

@mdopp I'd prefer to have it in a separate PR, however, I'd appreciate feedback from @mchwalisz first, before opening further PRs.