lex and lexgen: Allow encodings in Lexicon files to be arrays of strings

[mikekasprzak]

As per bluesky-social/atproto-website#426 and bluesky-social/atproto#3986, this PR adds initial support for arrays of encodings in Lexicon files. See the following example:

{
  "lexicon": 1,
  "id": "org.w3.activitypub.putInbox",
  "defs": {
    "main": {
      "type": "procedure",
      "input": {
        "encoding": [
          "application/json",
          "application/activity+json",
          "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\""
        ],
        "schema": { "type": "ref", "ref": "org.w3.activitypub.object" }
      },
      "output": {
        "encoding": "application/activity+json",
        "schema": { "type": "ref", "ref": "org.w3.activitypub.object" }
      }
    }
  }
}

In essence, this PR stops lexgen from emitting errors when an array of encodings is found in a Lexicon file. This feature is currently of no use to the Go codebase, as Lexicons used here all have only a single encoding. That said, it ensures interoperability should bluesky-social/atproto-website#426 get approved.