staging: Deploy maven index exporter to extract export.fld files

per maven repository.

For each maven repository configured in the configuration key
maven_index_exporter::repositories, this installs:

• one timer maven_index_exporter@.timer and its associated service maven_index_exporter@.service
• one run_maven_index_exporter.sh script to compute export.fld (the script is triggered by the service)

For now, this also installs 2 maven repositories to crawl:

• clojars
• maven-central

Related to T3746