Range function for the manifest entries, so we can reduce the
appearances of `.Paths()` usages like:

```go
for _, path := range m.Paths() {
  digest, ok := m.DigestFor(path)
  if !ok {
    // this will never happen
    return nil, fmt.Errorf("digest not found for path: %s", path)
  }
  // do something with path and digest...
}
```

into:

```go
m.Range(func(path string, digest manifest.Digest) error {
  // do something with path and digest...
})
```

---------

Co-authored-by: Saquib Mian <smian@buf.build>