Use aws_credentials for S3 file system if Access Key ID is missing

[mbklein]

Resolves #2341. Tested for the following cases:

• EC2 with instance role
• ECS with task role
• AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables
• Access Key ID and Secret Access Key provided via the FileSystem form

I ran into an issue trying to test with the aws_credentials_file provider, which uses the canonical AWS profile configuration files (~/.aws/config and ~/.aws/credentials). The error was coming from the eini parser, and I couldn't come up with a version of my config file that would satisfy it. Every other ini-file-compatible AWS client seems to work with my configuration just fine, so I'm not sure what can be done besides try to fix it upstream.

I also made the Access Key ID and Secret Access Key fields in the FileSystem form optional, but if either one is present, the other has to be as well in order for the changeset to validate.

[CLAassistant]

All committers have signed the CLA.

[mbklein]

@josevalim I've refactored things a little bit and pushed to an alternate branch for comparison. I think it's a more elegant solution and solves the two issues above.

The new option:

• Moves ensure_credentials/1 from S3.Client to S3 and makes it public
• Adds validate_credentials/1 so that the S3 changeset will only validate if both Access Key ID and Secret Key are supplied by the user or credentials exist in the environment at the time the FileSystem is added
• Removes Livebook.Utils.validate_mutual_inclusion/2 because it's no longer needed

If you agree that it's the better approach I'll merge it into this PR branch.

[mbklein]

PR branch is now up to date with all comments thus far

[jonatanklosko]

Suggested change

	@type credentials_t :: %{
	@type credentials :: %{

[mbklein]

I originally called the type credentials, but I wasn't clear on whether a module could contain a type and a function with the same name.

[josevalim]

This looks super clean! I think we are almost there. :)

[jonatanklosko]

LGTM!

[jonatanklosko]

@josevalim we should update FFS to accept the token and pass it from kino, right?

[github-actions]

Uffizzi Preview deployment-40863 was deleted.

[josevalim]

@jonatanklosko yes, I will ask @philss to work on it :)

[josevalim]

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