Pass initial configuration to additional configuration override.

[kflorence]

Pull Request Checklist

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• Have you updated the documentation?
• Have you added tests for any changed functionality?

Fixes

Fixes #2448

Purpose

Allow additionalConfiguration access to the initial application configuration which it may override.

Background Context

I had a use-case where I needed to reference values in the initial application configuration in order to implement my override logic. Currently this would require explicitly compiling the initial application configuration in the same way it is already compiled in LagomApplication inside of my additional configuration implementation. See #2448 for more details.

This PR still requires documentation updates and test coverage. I would also like guidance on deprecation: should we allow both methods to exist? If not, what should be provided in since? I have stubbed it out with 1.6.0 for now.

[ignasi35]

Thanks @kflorence! Left a few comments.

[kflorence]

@ignasi35 I will look into test coverage and documentation soon -- feel free to look over the method documentation to see if it is somewhat aligned with how this should be documented in the Lagom documentation. I am also not intimately familiar with Lagom tests -- so any pointers on where additional tests should go would be helpful (otherwise I will try to figure it out when I have some spare time).