[0.19] Return filtering label values as map

[rh-appservices-perf]

Backport: #2555

Fixes Issue

Fixes the issue that users might experience when they have multiple filtering labels values and many datasets.

Currently the filteringLabelValues endpoint returns for each dataset an object with the values of all filtering labels (defined in the schema on that dataset) - this could end up in loading a lot of data and making the browser hanging!

Changes proposed

• Add new endpoint to avoid introducing breaking change (/test/{0}/filteringLabels)
• Return a map where the key is the name of the label and the value is the list of all possible values for that label (across all datasets)
• Adapt the UI to manage this result structure
• Mark as deprecated the old endpoint

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.