Support for subdirectories in results-dir

[julioz]

Author the user story for this feature

As an engineer, I want to organize results from different flank runs so I can organize and simplify navigation in my Google Cloud Storage bucket.

On our pipelines we run multiple flank setups for different apps and use Fladle for the integration with Gradle.
I have configured each of the fladle blocks with the lines similar to the following:

fladle {
  ...
  resultsDir = "${readFlankResultDirectoryName()}/my-app-test-run-results"
}

In order to have multiple separate directories under the GCS bucket so that the test result artifacts (JUnitReport, MatrixResults, CostReport) don’t overwrite each other.

Describe the solution

Assuming I have 2 flank runs in a single pipeline, what I want to achieve is

gs://<bucket-name>/<pipeline-run>/<flank-run-1>/<artifact.txt>
gs://<bucket-name>/<pipeline-run>/<flank-run-2>/<artifact.txt>

Describe alternatives considered

What I could do as a workaround is to define a namespace to add as part of my bucket directory name.

Right now I still have to do something like

gs://<bucket-name>/<pipeline-run-flank-run-1>/<artifact.txt>
gs://<bucket-name>/<pipeline-run-flank-run-2>/<artifact.txt>

This is not optimal for dev. experience within my company because engineers have to search for all these directories on the root of my bucket that happen to be related. That is why I would rather organize each pipeline run into a single directory with nested directories for each flank run.

[piotradamczyk5]

I made similar logic in #1292, so maybe we could generalize it a little bit