Flow rate unit conversions and device class

[elupus]

Proposed change

• Add ability to convert between flow rates
• Add device class for flow rates
• Correct units for flow rates from ft³/m to ft³/min

Based on https://github.com/home-assistant/architecture/discussions/915

Missing (help accepted):

• [x] frontend icon PR
• [x] number+sensor dev docs PR
• [x] number+sensor regular docs PR

Type of change

• [ ] Dependency upgrade
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New integration (thank you!)
• [x] New feature (which adds functionality to an existing integration)
• [ ] Deprecation (breaking change to happen in the future)
• [ ] 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: #93828, #93828, #85905
• Link to documentation pull request: https://github.com/home-assistant/developers.home-assistant/pull/2034 https://github.com/home-assistant/home-assistant.io/pull/30748 https://github.com/home-assistant/frontend/pull/19344

Checklist

• [ ] The code change is tested and works locally.
• [x] Local tests pass. Your PR cannot be merged unless tests pass
• [x] There is no commented out code in this PR.
• [x] I have followed the development checklist
• [x] I have followed the perfect PR recommendations
• [x] The code has been formatted using Ruff (ruff format homeassistant tests)
• [x] 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.

To help with the load of incoming pull requests:

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

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[home-assistant[bot]]

Hey there @home-assistant/core, @shulyaka, mind taking a look at this pull request as it has been labeled with an integration (number) you are listed as a code owner for? Thanks!

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[home-assistant[bot]]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks :+1:

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