Resolve AwsClientCredentials if not provided

[Schwusch]

Partially resolves #160.
Tries to resolve credentials from these places if no credentials are provided:

• Environment Variables
• Credential profiles file at the default location (~/.aws/credentials) or AWS_SHARED_CREDENTIALS_FILE if set

Does not resolve from these sources:

• Credentials delivered through the Amazon EC2 container service if AWS_CONTAINER_CREDENTIALS_RELATIVE_URI" environment variable is set and security manager has permission to access the variable,
• Instance profile credentials delivered through the Amazon EC2 metadata service
• Web Identity Token credentials from the environment or container.

[isoos]

Looks good! Please fix formatting.

Not urgent, but maybe in the future resolve() could also use a pre-populated value too.

[Schwusch]

use a pre-populated value too

Do you mean partial credentials, filling in the rest? Would it override or be overridden?

[isoos]

Do you mean partial credentials, filling in the rest? Would it override or be overridden?

resolve() is almost like a singleton. If it gets a "setter" (we can call it setGlobalCredential or setResolveInstance) then an app can set it up as a "global" variable and downstream code can just use it.

[Schwusch]

If it gets a "setter" then an app can set it up as a "global" variable

I'm not certain how that would look, so I didn't add it this time.

[isoos]

I'm not certain how that would look, so I didn't add it this time.

Yeah, this can be figured out later.