[FHIR] Support configDropins

[lmsurpre]

Liberty supports configuring the server through configDropins. The IBM FHIR Server ships with some sample configDropins for common configuration tweaks to the server.

For example

• datasource definitions for db2, postgresql, and derby.
• connection pool info
• authorization tweaks (e.g. JWT validation)

In our current helm chart, we expose the datasource definitions via a helm value that points to a named template and we let the user override this template.

What would be nice is to have a way for users to pass other configDropins to the helm chart to configure their server.
Maybe a single extra value like configOverrides that takes a named template and will be placed in the configDropins/overrides dir alongside the datasource one?