This commit introduces a new configuration option, `custom_carbon_intensity_g_co2e_kwh`,
allowing you to specify a direct carbon intensity value (in gCO2e/kWh) for your
energy consumption.

When this value is provided (either via the `.codecarbon.config` file or the
`CODECARBON_CUSTOM_CARBON_INTENSITY_G_CO2E_KWH` environment variable),
it overrides all other methods of determining carbon intensity, including
cloud provider data, CO2 Signal API, and default geographical energy mixes.

Key changes include:
- Modifications to `codecarbon/core/config.py` to recognize the new
  configuration parameter (though existing mechanisms were largely sufficient).
- Updates to `codecarbon/emissions_tracker.py` (specifically
  `BaseEmissionsTracker`) to read, validate (must be a positive float),
  and pass the custom intensity to the `Emissions` class.
- Updates to `codecarbon/core/emissions.py` to use this custom intensity
  value in `get_cloud_emissions` and `get_private_infra_emissions` if provided,
  bypassing other data lookups.
- Comprehensive unit tests added to:
    - `tests/core/test_config.py` for configuration loading.
    - `tests/test_tracker.py` for validation and initialization logic in
      `BaseEmissionsTracker`.
    - `tests/core/test_emissions.py` for the `Emissions` class calculation
      logic using the custom value and fallback mechanisms.

This feature provides you with greater flexibility and accuracy in
reporting emissions, especially for on-premise setups with specific energy
mixes or when default data sources are not representative.