Add external reader support in pkl-gradle

[HT154]

No description provided.

[bioball]

@HT154 can we add some test coverage for this?

I think this would involve moving TestExternalReaderProcess to pkl-commons-test, and adding a corresponding test to org.pkl.gradle.EvaluatorsTest.

Hint: the Groovy syntax for maps is [key: value].
External readers would look something like:

externalModuleReaders = ["foo": new org.pkl.gradle.ExternalReader("bar")]

[netvl]

Is there a material difference between null arguments and empty list of arguments? Because if not, I would recommend not marking this as nullable and instead use an empty list.

[netvl]

These should be marked as @Input properties, not @Nested. The @Nested annotation in Gradle is used to make Gradle process values of a map property as beans. All @Nested properties are treated as @Input, so it will work as is, but semantically values of this map are just data carriers without any annotations which do not require further processing, so I think that @Input would be more appropriate here.

[HT154]

Thanks @netvl! Do you know of any documentation that covers these subtleties? I wasn't able to find anything when I searched.