feat: Refactor GraphQL and REST API clients for EnergyZero

[klaasnicolaas]

Proposed change

• Added GraphQLClient for fetching electricity and gas prices via GraphQL API.
• Introduced RESTClient for retrieving electricity and gas prices through REST API.
• Updated const.py to define PriceType and Interval enums.
• Refactored EnergyZero class to support backend selection (REST or GraphQL).
• Enhanced error handling in exceptions.py for better clarity and data management.
• Updated models.py to streamline price data handling and introduced from_rest_dict method for REST API responses.
• Removed legacy methods for fetching prices, consolidating functionality into new API clients.

Additional information

• This PR fixes or closes issue: fixes #

Checklist

• I have updated the documentation if needed.
• I have updated the tests if needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3fec3f9) to head (958e54c).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #935   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            5         9    +4     
  Lines          305       385   +80     
  Branches        25        44   +19     
=========================================
+ Hits           305       385   +80     

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