Enable override of version at SQL level

[gbartolini]

Currently the version of the extension in the defaul tests is fixed and based on the version of the package. We should enable to customise this value.

[gbartolini]

Something like this?

  versions = {
    bookworm = {
      "18" = {
        // renovate: suite=bookworm-pgdg depName=postgresql-18-pgvector
        package = "0.8.2-1.pgdg12+1"
        // renovate: TODO
        sql = "0.8.2"
      }
    }
    trixie = {
      "18" = {
        // renovate: suite=trixie-pgdg depName=postgresql-18-pgvector
        package = "0.8.2-1.pgdg13+1"
        // renovate: TODO
        sql = "0.8.2"
    }
  }