Add config flow

[tomreay]

Migrate to a config flow so the integration can be configured via the UI. It also means the user only needs to supply an API key as a list of budgets, categories and accounts can then be shown to the user.

This is a breaking change and users would need to remove the configuration.yaml setup and add it via the UI instead.

[wxt9861]

Hi @tomreay, thank you this PR.

I have been thinking about this and how to best handle this. There are 2 schools of though with how integrations are configured with some people preferring config flows while others wanting to keep YAML. I see the benefit of both, and I am trying to figure out what to do with this component.

My current thought is to create another branch which will retain yaml configuration and keep that branch of v0.x releases. This PR would be merged into master and versions from master branch will become v1. This way future functionality not related to configuration can be merged into both branches and maintain parity. Users can choose to say on v0 if they prefer yaml or move to v1 releases.

I am not sure if this is a good idea... input welcome.

[tomreay]

It's not a bad approach but I think would add a lot of maintenance overhead - the general direction of Home Assistant seems to be UI configuration as it's much more beginner friendly. As long as all the options that were in YAML are available in the UI it seems ok to me