Add GetNodeByOID which keeps index

[stampflit]

This function looks up a node by oid and also returns the part of the oid that have not been used to look up the node. The usecase for this are oids of fields in a table. E.g. IF-MIB::ifDescr.4 (.1.3.6.1.2.1.2.2.1.2.4) will return the oid of ifDescr and a index of 4.

This functionality is hard to just implement anywhere else as the code in smi/node.go interacts with smi/internal. So this functionality is tied closely to gosmi and I can't just implement my own function for this in my own module.

I don't really like the function names and the signature, but I also don't have better ones. I'll gladly change them upon suggestion.

[sleepinggenius2]

I'm confused about why this couldn't be implemented directly in your own code. This PR as written appears to be functionally equivalent to:

func GetNodeByOIDWithIndex(oid types.Oid) (node gosmi.SmiNode, index []types.SmiSubId, err error) {
  node, err := gosmi.GetNodeByOID(oid)
  if err != nil {
    return node, nil, err
  }
  return node, oid[len(node.Oid):], nil
}