Add config flow to trend

[jpbede]

Proposed change

Add a config flow for the trend integration. When merged trend could be set up either through configuration.yaml or through the UI.

The migration and deprecation of the yaml configuration follows in a later PR to keep this small.

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)
• [x] 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:
• Link to documentation pull request: https://github.com/home-assistant/home-assistant.io/pull/28813

Checklist

• [x] 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 Black (black --fast homeassistant tests)
• [x] Tests have been added to verify that the new code works.

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

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

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

• [x] 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:

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

[jpbede]

Hmm the PR still not gets labeled.

[joostlek]

Yea, webhooks failed yesterday, I'll give it the appropriate labels

[jpbede]

Thanks @joostlek 👍

[jpbede]

Converted to draft again, need to clean up the tests first

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[jpbede]

Thanks for your review @gjohansson-ST

[gjohansson-ST]

We need a test for yaml setup which also includes setting the entity_id Probably duplicating the test_basic_trend test into test_basic_trend_yaml_setup and add entity_id

[jpbede]

Thanks @gjohansson-ST for the review and adding the test