Alter manifest API output to include HTTP headers

[alanshaw]

Per storacha/RFC#79

Should be a relatively simple refactor to change the []string into a []Source where Source looks something like:

type Source struct {
  URL string
  Headers map[string]string
}

spade/apitypes/sppiecemanifest.go

Line 10 in 9e6ba24

Sources []string `json:"sources"`