feat: support sql version in metadata files

[GabriFedi97]

Summary

Adds support for tracking PostgreSQL extension SQL versions separately from Debian package versions, enabling accurate version verification in E2E tests.

Changes

• Metadata structure: Extensions now define both package (Debian version) and sql (PostgreSQL catalog version) in metadata.hcl
• Image annotations: New io.cloudnativepg.image.sql.version label added to container images
• E2E testing: Chainsaw tests verify extension version against the new sql value
• Renovate support: Updated patterns to track both package and SQL versions automatically
• Documentation: Updated templates and contributing guide to explain version semantics

Example

versions = {
  bookworm = {
    "18" = {
      package = "0.8.2-1.pgdg12+1"  // Full Debian package version
      sql     = "0.8.2"              // Version in PostgreSQL catalog
    }
  }
}

The SQL version is optional and only required for extensions using CREATE EXTENSION (create_extension = true).

closes #145