Migration Configuration File Inheritance

[ClintonWB-EBSCO]

Currently, migration files are a single file, containing both client metadata and configuration.

In order to support multi-tenant migrations, such as consortia migrations, we should support sharing configurations between clients, with customization per-client in IDs, URLs, and in some custom processing.

To this end, I propose that we allow a configuration file to support a block:

"inherits": [
   "relative/path/to/a",
   "relative/path/to/b"
]

Similar to other configurations (such as Terraform).

The behavior would be to simply Apply each object in turn, at full depth, combining and overwriting properties as they appear.

While this would reduce clarity a bit, and add a bit of processing, I believe it will be worth it in support - and will also help with keeping multiple configuration files for a single client, if desired, without duplicating information.