[develop | DONE | CHANGED]: data sources landing: Get and parse number of collected fulltexts from a new stats api.

1. env-properties.ts: Added property pdfStatisticsAPIURL?: string;
2. environment.ts: Added pdfStatisticsAPIURL property in all environments.
3. dataProvider.service.ts: Uncomment method "getCollectedFulltexts()".
4. dataProvider.component.ts: Call method "getCollectedFulltexts()" instead of "getDataProviderAggregationStatus()".

explore/src/app/openaireLibrary

@@ -1 +1 @@
-Subproject commit 68c352d3d275209d52bc3198dad7482ee21d193d
+Subproject commit 7f9117eccb43f1e29a84b6e7e6d4494da6a34384