Support `~/.aws/config` file parsing

[AliSoftware]

What

Especially so that even if region is not defined in ~/.aws/credentials but is defined in ~/.aws/config, we can still create AWSCredentials with that region value.

Fixes https://github.com/Automattic/hostmgr/issues/83

How

The parsing which was previously done by the AWSCredentialsFileParser class is now handled via:

• A AWSProfileConfig class, used to parse any AWS user config file—be it ~/.aws/credentials as before, or ~/.aws/config (new)—and collect all the key/value pairs of each profile found in those files.
• The AWSCredentials class can then now be constructed from an array of those AWSProfileConfig objects, combining the values for a profile parsed from different files if needed

I've then updated the tests to account for more scenarios combining those.

[!Note]
While the ~/.aws/credentials and ~/.aws/config files are very similar, they use slightly different format for profile sections, with the ~/.aws/config file listing profiles as [profile <profilename>]—except for the special [default] one—and potentially having other section types, like [services <…>] or [sso-session <…>] too.

[AliSoftware]

ping @jkmassel for another review 🙂 (especially to lift the "change requested" blocker if you agree that I addressed all your latest feedback)