feat: subseries api for red

[jennifer-richards]

Adds API calls subseriesList and subseriesRetrieve. The former retrieves all subseries docs, optionally filtering by type (bcp, std, and fyi are the allowed values). The latter retrieves a single subseries doc.

This PR does not yet add the links from RFC documents to the subseries they belong to. The is_also value for each document is still faked and always an empty list. That will be a separate PR.

The subseries doc schema for a subseries document looks like this, with the List returning a list of these records. The contents value is a list of the RFCs contained in the document. The schema of the entries in contents is the same as the docList endpoint that retrieves a list of RFCs. This is still incomplete the same way that was incomplete - e.g., the formats lists are faked. I'm planning to straighten that out in the shared code separately also.

{
  "name": "bcp243",
  "type": "bcp",
  "contents": [
    {
      "number": 9813,
      "title": "Operational Considerations for Using TLS Pre-Shared Keys (TLS-PSKs) with RADIUS",
      "published": "2025-07-11",
      "status": {
        "slug": "bcp",
        "name": "best current practice"
      },
      "pages": 20,
      "authors": [
        {
          "person": 108624,
          "name": "Alan DeKok",
          "email": "alan.dekok@inkbridge.io",
          "affiliation": "InkBridge Networks",
          "country": ""
        }
      ],
      "group": {
        "acronym": "radext",
        "name": "RADIUS EXTensions"
      },
      "area": {
        "acronym": "sec",
        "name": "Security Area"
      },
      "stream": {
        "slug": "ietf",
        "name": "IETF",
        "desc": "Internet Engineering Task Force (IETF)"
      },
      "identifiers": [
        {
          "type": "doi",
          "value": "10.17487/RFC9813"
        }
      ],
      "obsoletes": [],
      "obsoleted_by": [],
      "updates": [],
      "updated_by": [],
      "is_also": [],
      "see_also": [],
      "draft": {
        "id": 119429,
        "name": "draft-ietf-radext-tls-psk",
        "title": "Operational Considerations for RADIUS and TLS-PSK"
      },
      "abstract": "This document provides implementation and operational considerations for using TLS Pre-Shared Keys (TLS-PSKs) with RADIUS/TLS (RFC 6614) and RADIUS/DTLS (RFC 7360).  The purpose of the document is to help smooth the operational transition from the use of RADIUS/UDP to RADIUS/TLS.",
      "formats": [
        "txt",
        "xml"
      ],
      "keywords": [
        "keyword"
      ],
      "errata": []
    }
  ]
}