WIP use ServiceLoader to load credential factories

[chriskilding]

This PR enables the dynamic discovery of credential type factories via the Java ServiceLoader.

It lays the groundwork for the Secrets Manager Credentials Provider to support custom credential types (such as GitHub or BrowserStack credentials). This will be achieved by creating downstream plugins for each of these credential types; each downstream plugin will contain an AwsCredentialsFactory implementation.

As a useful side benefit, breaking the credential type factories out into their own plugins should also make the test suites for each one simpler and quicker.

Testing done

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