Added feature to fetch s3 credential from aws provider chain

[shhnwz]

Introduced a new boolean server option "use_credential_provider", by default its set to FALSE. In case of TRUE, AWS credentials will be fetched through DefaultAWSCredentialProviderChain, and it follows the following credential probe order.

Default Credential Provider Chain

The default credential provider chain does the following:

    Checks your environment variables for AWS Credentials
    Checks your $HOME/.aws/credentials file for a profile and credentials
    Contacts and logs in to a trusted identity provider (Cognito, Login with Amazon, Facebook, Google). The sdk looks for the login information to these providers either on the environment variables: AWS_ROLE_ARN, AWS_WEB_IDENTITY_TOKEN_FILE, AWS_ROLE_SESSION_NAME. Or on a profile in your $HOME/.aws/credentials.
    Checks for an external method set as part of a profile on $HOME/.aws/config to generate or look up credentials that isn't directly supported by AWS.
    Contacts the ECS TaskRoleCredentialsProvider service to request credentials if Environment variable AWS_CONTAINER_CREDENTIALS_RELATIVE_URI has been set.
    Contacts the EC2MetadataInstanceProfileCredentialsProvider service to request credentials if AWS_EC2_METADATA_DISABLED is NOT set to ON.`

[ChuckHend]

This addresses #28

[shhnwz]

Test Diffs with the with and without code changes.
regression_diff.zip

[ChuckHend]

@hrkuma, any chance we could get a review or discussion on this?

[ChuckHend]

We'd love to see this feature get moved forward and merged. In the meantime, we'll be running using this feature off this fork.