early-boot-config: Use local file in sequence with other user data retrieval methods

[zmrow]

Currently, using a local file (/etc/early-boot-config/user-data) for user data is supported in aws-dev. It is an implementer of the PlatformDataProvider trait. In main(), we decide to use either local file or another provider because we expect a PlatformDataProvider trait object.

We should be able to use local file and another provider's methods. In order to do this, I think we should move local file support to a shared function that returns Result<Option<SettingsJson>> so it can be used in sequence with other methods.