add health checks to biosamples

Currently [biosamples health checks](https://www.ebi.ac.uk/biosamples/actuator/health) check that mongo is up and the app is up.

We should add checks to verify connectivity to other dependencies:
- [ ] biovalidator
- [ ] solr

Probably this could be integrated with official EBI monitoring.

Start by opening the existing Biosamples health endpoint at https://www.ebi.ac.uk/biosamples/actuator/health and identify how Mongo and application status are currently reported. Add connectivity checks for biovalidator and Solr, then verify the endpoint reports their status; the possible integration with official EBI monitoring remains an open requirement.

Written by the indexing model from the issue text.