Add adminX_code in results

[papjul]

Previous discussion: open-meteo/open-meteo#355

Reopening here as I opened it on the wrong repo.

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Initial message:

Just like we have country_code (IT, FR, etc), would it be possible to add admin1_code, admin2_code, etc?

This will be useful for #351 as Météo-France emits alerts by admin2 and using the admin2_code is much easier than having to parse the admin2 translatable field.

Thank you!