Fix input snapshotting error when using test clusters cliSetup

[mark-vieira]

This PR fixes an error when attempting to use the cliSetup configuration option on a test cluster. The root cause was that our LazyPropertyList did not properly handle collection types that were themselves complex types with their own @Input annotations. This PR introduces a getFlatNormalizedCollection() method which allows Gradle's @Nested annotation to work properly.

Example error:

Unable to store input properties for task ':plugins:ingest-attachment:integTestRunner'. Property 'clusters.integTest$0.nodes.$0.cliSetup.$0.value' with value 'org.elasticsearch.gradle.testclusters.ElasticsearchNode$CliEntry@262bda8d' cannot be serialized.
> Could not serialize value of type ElasticsearchNode.CliEntry
  > org.elasticsearch.gradle.testclusters.ElasticsearchNode$CliEntry

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Build)

[rjernst]

LGTM