Feat#86 credentials

[rle-earthdaily]

Add various credentials loading methods in the Auth class, all centralized in the Auth.from_credentials method, which is then called by the earthdaily.earthdatastore.EarthDataStore function. The idea is to stop users from importing the Auth class directly and propose a consistent and simple way of loading credentials.

A few notable points:

• No breaking change, but a deprecation warning for people that use the Auth constructor to load credentials from a JSON file
• The new authentification methods are tested in tests/test_auth.py
• Examples now use the new EarthDataStore function
• Unit tests now use the new EarthDataStore function
• Remove the Venus example, after a discussion with @nkarasiak
• More information in the README for credentials handling

[nkarasiak]

What a great commit ! I merge it on dev. We need to talk a bit about the copy-credentials-template before merging to main.