Add Doppler secrets integration.

[cuelindar]

Add DopplerConfiguration to access Doppler secrets as a Configuration.
See: https://www.doppler.com/
Update pyproject to includes doppler in cloud feature.
Update README to include DopplerConfiguration.
Add python 3.13 to supported environments.

[cuelindar]

@tr11 First, thank you for creating/maintaining this package!

This is my first time using Hatch... I managed to get tests to run, and nothing I'm adding has any lint or pytest errors. I was having issues getting hatch run testing:test to run all versions of the intrepreter; it would run 3.8 and then stop and report... I got it to run pytest on all versions by locally removing the addopts line from pyproject:

• pyproject.toml#L166

[tr11]

Thanks for the PR. I'll have some time earlier next week and will merge it then.