AmazonS3Builder::from_env to populate credentials from environment

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

There are various common environment variables for configuring AWS credentials, e.g. AWS_SECRET_ACCESS_KEY, AWS_REGION, etc... We should add a from_env method that respects these

Describe the solution you'd like

Describe alternatives you've considered

Additional context

[amrltqt]

I can help on this one.

Is it better to bring a full support of the builder properties through environment or just the minimum to leverage AWS_* regular environment setup?

Solution 1 we bring everything from environment but we need to introduce new variables to match each builder properties.

let object_store = AmazonS3Builder::from_env().build();

// or even ?
let object_store = AmazonS3::from_env();

Solution 2 we match existing AWS variable and provide support for AWS_* variables described there

let object_store = AmazonS3Builder::from_env()
    .with_bucket_name("the-bucket-name")
    .with_allow_http(true);

Solution 3 we support only the 4 variables used in integration test for now
(those ones: https://github.com/apache/arrow-rs/blob/master/object_store/src/aws/mod.rs#

Which one is the most suitable for you?

[tustvold]

Thank you for volunteering, I think option 2 makes the most sense. Specifically where one of the builder options has a standard AWS_ environment variable, we support that.

The idea is to get people up and running quickly, much like the standard AWS SDKs, not necessarily to expose the full set of customisation supported by this crate as environment variables, nor necessarily to support the full set of configuration supported by the CLI (e.g. I don't anticipate supporting AWS_CONFIG for example).