allow subgraph manifest to refer to `spkg.io`

[saihaj]

Extending #1641 in the subgraph manifest file if user updates to have spkg.io/**/*.spkg at the graph build step it should download the file and pin that to IPFS. Will be more involved and should be done after #1642 is merged.

[kcraftcode]

@saihaj I want to work with you.