Optimized implementations of References for DagPbCodec and DagJsonCodec

[rklaehn]

Currently, DagCbor References scrapes references from a blob very efficiently. But DagPbCodec and DagJsonCodec does not. It creates a temporary IPLD AST just to scrape the links from that and throw away the rest.

Especially for DagPbCodec this should be easy, since links are all in one part.

// An IPFS MerkleDAG Node
message PBNode {
  // refs to other objects
  repeated PBLink Links = 2;
  // opaque user data
  bytes Data = 1;
}