chart: support per-database URLs for v3 migrations

[marcopiraccini]

Summary

• support exact per-service ICC database URLs while retaining the shared database_url fallback
• preserve existing database names, credentials, and permissions during v3 to v4 migrations
• keep chart 4.1 Secret aliases compatible and require a Workflow URL only when Workflow is enabled
• update the installation and migration documentation for chart 4.2.0
• add CI coverage for shared, exact, mixed, Traffic Inspector fallback, and Workflow database configurations

Why

Chart v3 accepted a complete connection URL for each ICC database. The v4 chart only accepted one base URL and derived fixed database names, which could not represent running v3 installations that use separate roles or custom database names.

This change lets those installations upgrade without renaming databases or changing credentials. ICC continues to apply schema migrations automatically when it starts.

Tests

• bash test/database-urls.sh
• helm lint with shared and per-database values
• helm template with default and optional services
• helm package chart --destination /tmp

[mcollina]

lgtm