Add schema build config

[danielrbradley]

When calling the java-gen program, we used the option --build gradle-nexus to also generate the gradle build files. Internally, this set the configuration:

  "buildFiles": "gradle",
  "gradleNexusPublishPluginVersion": "2.0.0",

When moving to running java SDK generation via pulumi package gen-sdk, we can't set CLI arguments and don't want to have to hard code plugin versions in our schemas.

Therefore, it would be nice to have a new language configuration key to mirror the old CLI argument behaviour so we can just add to our schemas:

  "build": "gradle-nexus",

See also: pulumi/pulumi-azure-native#3776