Add option to output HTTP link to Singularity container

[itrujnara]

Issue

If a Singularity container is generated with the CLI, only a link with oras//: is provided. Due to compatibility issues, many Singularity-based systems still require an HTTPS link to the container blob. For this reason, nf-core only accepts HTTPS for Singularity, also when Wave is used. The web app solves this issue, but sometimes using the CLI is necessary or preferred. Obtaining such links is currently a highly manual process.

Reproduction

wave --conda python --singularity --freeze. An ORAS link is generated.

Proposed solution

Add an --http flag that returns an HTTP link to the blob if --singularity is used and does nothing otherwise.

[ewels]

x-ref:

• [Feature] Download Singularity image without having singularity installed locally wave#443
• Add API endpoint to fetch the URIs of layers of image wave#452

These were more about adding API endpoints to retrieve Singularity HTTPS container images. But likely amounts to the same thing.