feat: support required extensions

[mbiagetti]

initial support for extensions dependencies

introduce a new field in the metadata spec to define a list of required extensions

supposing a metadata file like

metadata = {
  name                     = "pgrouting"
  sql_name                 = "pgrouting"
  image_name               = "pgrouting"
  shared_preload_libraries = []
  extension_control_path   = []
  dynamic_library_path     = []
  ld_library_path          = []
  auto_update_os_libs      = false
  required_extensions      = [
      "postgis",
  ]

  versions = {
    bookworm = {
      "18" = ":4.0.0+dfsg-1.pgdg12+1"
    }
    trixie = {
      "18" = "4.0.0+dfsg-1.pgdg13+1"
    }
  }
}

the command generate-testing-values command will generate the proper extensions like

dagger \
call \
-m ./dagger/maintenance/ \
generate-testing-values --target pgrouting --extension-image ghcr.io/cloudnative-pg/pgrouting:4.0.0-18-bookworm export --path=pgrouting/values.yaml

extensions:
    - dynamic_library_path: []
      extension_control_path: []
      image:
        reference: ghcr.io/cloudnative-pg/pgrouting:4.0.0-18-bookworm
      ld_library_path: []
      name: pgrouting
    - dynamic_library_path: []
      extension_control_path: []
      image:
        reference: ghcr.io/cloudnative-pg/postgis-extension:3.6.1-18-bookworm
      ld_library_path:
        - /system
      name: postgis
name: pgrouting
pg_image: ghcr.io/cloudnative-pg/postgresql:18-minimal-bookworm
sql_name: pgrouting
version: 4.0.0

closes #18