Add UniqueID to AsusWRT config entry

[ollo69]

Proposed change

This PR is to add UniqueID to AsusWRT config entry and allow configuration of multiple instance. It use as UniqueID the device Mac Address retrieved from nvram
To not break compatibility with existing configuration and in case that we are not able to retrieve mac-address from nvram, I implemented this logic in config flow:

• if a device without UniqueID is configured, flow abort with error "single instance allowed"
• if during configuration is not possible to determinate device UniqueID there are 2 options:
  • if another device with a valid UniqueID is configured, flow is aborted
  • if no devices are configured, configuration complete without UniqueID (and we come back to check at first point if user try to configure a second device)

I do not implemented any automatic migration of existing configurations because it would cause names change for existing entities.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

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To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[probot-home-assistant]

Hey there @kennedyshead, mind taking a look at this pull request as it has been labeled with an integration (asuswrt) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[MartinHjelmare]

Please address the comments in a new PR. Thanks!

[ollo69]

Created follow up PR to address requests