Adding build time generated files to `service` and `var`

[tpetracca]

I have a service for which at build time I generate files that I would like to be included in service. Currently per the README we only package merge in the contents of ${projectDir}/service.

Furthermore, the Java Service plugin will merge the entire contents of ${projectDir}/service and ${projectDir}/var into the package.

Because my files are generated at build time, I populate them in ${projectDir}/build/service and thus they don't get included in the distribution.

For now I can work around this by just dropping them at build time into ${projectDir}/service, but that feels janky. A similar workflow would make sense for things you want to build time generate and place in var. An existing use case for this would be configuration files generated by gofigure.

A benefit to this is that clean commands will then properly clear out stale files.

I don't feel strongly about the actual path it looks for things in, maybe ${projectDir}/build/sls/[service|var]?