feat: add support for named FHIR connections in Helm chart

[mechanik-daniel]

This pull request adds support for managing named FHIR connections via a Secret-backed connections.yml file in the FUME Helm chart. It introduces a new configuration section that allows users to define FHIR connection details in a YAML file, which is securely mounted into the backend deployment. The implementation includes validation to prevent misconfiguration and updates the documentation to guide users through the new setup.

Named FHIR Connections feature:

• Added a new fhirConnections section in values.yaml to enable, configure the mount path, and provide the YAML content for named FHIR connections. The file is stored in a Kubernetes Secret and mounted into the backend container. (helm/fume/values.yaml, helm/fume/templates/connections-secret.yaml, [1] [2] [3]
• Updated backend deployment to mount the connections.yml Secret at the specified path, set the FHIR_CONNECTIONS_FILE environment variable automatically, and ensure the deployment is reloaded when the Secret changes. (helm/fume/templates/backend-deployment.yaml, [1] [2]

Validation and conflict prevention:

• Added Helm template logic to validate that when fhirConnections.enabled is true, both mountPath and yaml are provided, and to ensure env.FHIR_CONNECTIONS_FILE does not conflict with the configured mount path. (helm/fume/templates/_helpers.tpl, helm/fume/templates/_helpers.tplR91-R101)
• Ensured the $env variable is initialized to avoid nil map errors during validation. (helm/fume/templates/_helpers.tpl, helm/fume/templates/_helpers.tplR73)

Documentation updates:

• Updated the README to document the new Named FHIR Connections feature, including usage instructions, example values, and deployment notes. (helm/fume/README.md, [1] [2]