feat(config): Add support for overriding framework config

[liamperritt]

Allow forked repos to override framework config without being impacted by upstream OSS config changes.

If a forked repo makes changes to the src/config folder to customise the framework for their purposes, upstream OSS changes to the files in the src/config folder risk impacting downstream forked repos.

This change adds an empty src/config_override folder where forked repos can add and change framework config without any risk of upstream OSS config changes impacting their customised config.

The proposal is that if there are any non-hidden files within src/config_override, then this folder is leveraged by the framework for configuration. Otherwise, the framework will fall back to src/config.

This has been manually tested against the sample job "Run 1 - Load and Schema Initialization":

• Test without config_override: https://e2-demo-field-eng.cloud.databricks.com/jobs/314656373533508/runs/660582748996883?o=1444828305810485
• Test with config_override: https://e2-demo-field-eng.cloud.databricks.com/jobs/314656373533508/runs/865818877149104?o=1444828305810485