Add support to download pre-releases as well for github

ATM pre-releases are skipped

```python
if rel.prerelease:
log.info("Release %s is prerelease; skipping", rel.tag_name)
continue
```

so we should add config to optionally enable them, and then enable for e.g. https://github.com/sylabs/singularity/ (on smaug:/mnt/datasets/datalad/ci/singularity)

Start from the GitHub release handling code containing the shown rel.prerelease check, then trace how provider configuration is represented and loaded. Add an optional prerelease setting and enable it for the referenced Singularity configuration; verify that prereleases remain skipped by default and are downloaded when enabled.

Written by the indexing model from the issue text.