Qualify the integration for HA's core repo

[danielgospodinow]

A set of quality requirements have to be met in order to commit this integration into the core Home Assistant repo.

Tasks:

• Slice the current codebase into pieces and merge the pieces separately into the core repo.
  • Prepare a PR with a config flow and a water heater platform first.
• Refactor codebase (maybe directly in PRs towards the core repo) to comply with style/guidelines with the example integrations like Mealie or airgradient.
  • Note: eldom_convector.py and eldom_boiler.py should be removed; i have to somehow move those abstractions in the pyeldom lib.

Context: https://discord.com/channels/1312381782942810192/1312386863947972720/1313617421168283720