Add more timeseries database clients to OpenWISP Monitoring

The project adds InfluxDB 2.0 and Elasticsearch as timeseries database backends to OpenWISP Monitoring by implementing the existing DatabaseClient. The core monitoring logic remains untouched since it already interacts through timeseries_db abstraction, making backend addition a clean isolated task. For InfluxDB 2.0, all chart queries in `queries.py` will be translated from InfluxQL to Flux. For Elasticsearch, queries will use `date_histogram` aggregations. Each backend gets dedicated GitHub Actions CI, deployment support via ansible-openwisp2 and docker-openwisp, and users can switch between all three backends through the `TIMESERIES_DATABASE` setting with full documentation coverage.