Adopt Secret to pgvector

[davidsbatista]

PostgreSQL connection string, read from an environment variable with the Secret component:

• adapted the tests to use the secret component
• added a custom from_dict method to the doc store class
• set some methods as staticmethod, since the self was not being used
• change the coverage config so that it actually shows the coverage over code and not test
• improved the documentation
• fixed some typos

[anakin87]

Hey! I'm not still an expert on the new Secret management.

I think your implementation should follow in some way that of UptrainEvaluator.

In the current PR, I see that the connection_string is resolved at initialization,
then the to_dict method includes it in serialization, leaking it.

I don't know what the best way to approach this is, and I would leave it to you to experiment/decide.
One possibility is to have a public connection_string that is serialized as Secret, without leaking it and another internal one, _connection_string, that is not serialized/is excluded by the to_dict method.

Let me know what you think and if you need help.

[davidsbatista]

thanks for your comments @anakin87 - I did some changes

[anakin87]

Great job!

I left a comment with a doubt. if it makes sense, please consider it.