[refractor]: decoupling sequential and parallel downloads while fetching SBOMs in github adapter

[viveksahu26]

This PR adds the following changes:

• Decoupled Sequential and Concurrent Fetching:

  • Previously, both sequential and concurrent modes relied on fetchSBOMFromReleases, which used concurrent downloading (3 goroutines) even for sequential fetching. This PR separates them:
    • Sequential mode now downloads SBOMs one-by-one, i.e. sequential processing.
    • Concurrent mode performs a concurrency limit of 3 downloads.

• Streamlined SBOM Fetching:

  • Earlier, the process involved two steps:
    • FindSBOMs collected metadata (e.g., name, size, download URL).
    • downloadSBOMs fetched content using those URLs.
  • Now, FindSBOMs handles both metadata retrieval and content downloading in a single step, storing []SBOMData (content and metadata). This eliminates the separate downloadSBOMs step, simplifying the pipeline.