Download default configs when deploying Purpur and Pufferfish

### Enhancement Type

Improve an existing feature

### Describe the enhancement

Since Purpur and Pufferfish are forks of Paper, it would be nice to download the same default configurations as for Paper. They are compatible because Purpur and Pufferfish have separate configuration files (`purpur.yml` / `pufferfish.yml`) for fork-specific features.

Essentially, we would simply have to extract this code into a reusable function/script and invoke it from `start-deployPaper`, `start-deployPurpur` and `start-deployPufferfish`.
https://github.com/itzg/docker-minecraft-server/blob/621bafb4f257da547a0e5837ad4400833fcee345/scripts/start-deployPaper#L45-L56

And we would have to think about what to do with this environment variable (keep the same name for forks, rename it, ...?): https://github.com/itzg/docker-minecraft-server/blob/621bafb4f257da547a0e5837ad4400833fcee345/scripts/start-deployPaper#L13

Start by reading scripts/start-deployPaper, especially lines 45-56 and the environment variable referenced near line 13. Trace start-deployPurpur and start-deployPufferfish, then determine how the shared default-config download should be invoked and how the environment variable should apply. Done means all three deployment paths handle the compatible Paper defaults consistently.

Written by the indexing model from the issue text.