Rewrite more robust UUID plugin

[robfitzgerald]

the current UUID output plugin expects queries that are tagged with {origin|destination}_vertex fields and looks up those values in a lookup table to append UUIDs. it fails if the vertex data is missing from the query.

let's refactor this so that instead, in configuration, we have

[[output_plugins]]
type = "UUID"
vertex_uuid_input_file = ""  # optional
edge_uuid_input_file = ""  # optional

during UuidOutputPlugin::process, we check for the following cases for each combination of origin/destination/edge/vertex:

match (query.get_origin_vertex(), self.vertex_uuids) {
  (Some(vertex_id), Some(vertex_uuids)): (Option<VertexId>, Option<Box<[String]>>) => { append_uuid() },
  _ => {} // do nothing, as opposed to failing here 
}