Make it easier to load additional config properties files at build time

[xtaixe]

Description
We have (generated) additional config properties files that we would like to load like application.properties. It would be really nice o be able to load them without having to define a ConfigSource and doing the SPI registration, etc.

See discussion at https://quarkusio.zulipchat.com/#narrow/stream/187030-users/topic/Additional.20config.20properties.20at.20build.20time

Implementation ideas
I guess some options are:

• Something similar to Spring's @PropertySource
• Define which additional files to load in a property in application.properties

[radcortez]

Just to clarify, this is for runtime, not build time, right? We need to be careful at build time since the resources may not be available at runtime, since the class path may be different.

[xtaixe]

@radcortez build time would work for our current use case, but runtime should be fine too. Not sure how resources work with Quarkus/GraalVM, but would be nice to not have to define additional parameters when building/running if the resource is already defined somewhere.

[radcortez]

Ok, I may have some ideas. I'll have to prototype and experiment with them first. I'll update later.