Make redirecting obsolete terms optional

[mbklein]

This PR adds support for fetching obsolete terms without redirecting, so the old label can be retrieved.

Changes:

• Add Authoritex.Record and Authoritex.SearchResult structs to return instead of bare maps
• Add opts \\ [] argument to Authoritex.fetch to allow for redirection (default: false)
• Update Getty authorities to pull obsolete label if needed
• Update LOC authorities to detect and handle obsolete terms
• Update FAST authorities to detect and handle obsolete IDs (e.g., http://id.worldcat.org/fast/fst01205331 replaced by http://id.worldcat.org/fast/1205331 despite still being the same term)

This will be a MAJOR version update, as it breaks both the calling interface, the shape of the return values, and the redirect behavior:

# Without auto-redirect
iex> Authoritex.fetch("http://vocab.getty.edu/aat/300423926")
{:ok,
  %{
    id: "http://vocab.getty.edu/aat/300423926",
    label: "eating fork",
    qualified_label: "eating fork",
    hint: nil,
    variants: [],
    related: [replaced_by: "https://vocab.getty.edu/aat/300043099"]
  }}

# With auto-redirect
iex> Authoritex.fetch("http://vocab.getty.edu/aat/300423926", redirect: true)
{:ok,
%Authoritex.Record{
  id: "http://vocab.getty.edu/aat/300043099",
  label: "forks (flatware)",
  qualified_label: "forks (flatware)",
  hint: nil,
  variants: ["fork (flatware)", "eating fork", "叉子", "vork", "prakijzers",
    "Gabeln (Essbestecke)", "tenedor"],
  related: [replaces: "http://vocab.getty.edu/aat/300423926"]
}}

I am very open to calling the added field something other than extra. I thought maybe flags or notes, but couldn't really find one I felt great about. related seems much better.

First step in fixing repodev_planning_and_docs#5783

[bmquinn]

Looks great, though I think extra is a little vague. Approving regardless because I can live with it, but what do you think about provenance or relations instead?