Support additional property sources during configuration

During a brainstorm for removing the explicit runner found in https://github.com/moditect/layrry-examples/tree/master/modular-tiles I thought it might be a good idea to provide the Layrry launcher with an addition source of properties that can be used for placeholder substitution, something akin to

`java -jar layrry-launcher.jar --layers-config layers.yml --properties layrry.properties`

The launcher would then read the properties file and place all properties into the Mustache scope, allowing placeholder substitutions. This could turn a somewhat static Layers config file into a “self updatable” config source, as [groupId, artifactId, version] could be provided by external properties. No need to provide a similar layers.yml file to update a version (say from groovy-json:3.0.5 to groovy-json:3.0.6) but also may handle artifact relocation.

Example: Groovy 3 is currently published with groupId = org.codehaus.groovy. Groovy 4 will change to groupId = org.apache.groovy.

A layers file that requires Groovy could declare a module definition as

"{{groovy.groupId}}:groovy-json:{{groovy.version}}"

Explicit definition of an additional properties source makes sense for locally launched applications but may pose a security risk with remotely launched applications (see #37) where you’d want the property source to come from the same trusted host (and adjacent location perhaps) as the layers config file. An implicit mode could be implemented

1. Given the path of the config layers file replace the filename extension with .properties. If a match is found, use it.
2. Given the path of the config layers file, locate a file named layrry.properties. If a match is found, use it.
3. If there are no matches in 1) and 2) then no implicit property sources will be used.

Of course, explicit declaration such as --properties my-props.properties win over implicit, such that if the explicit file is not found then no implicit search will be performed.

[gunnarmorling]

I’ve got a gut feeling that 3&2 > 4 > 1 is good enough

👍

if we need to loosen up at a later date then 4 > 2&3 > 1 is the way to go.

I still think it should then be 4 > 1 > 2&3, but let’s cross that bridge when we get there 😃

[gunnarmorling]

Done.