Use components for common params in OpenAPI spec

I noticed in the Omicron OpenAPI spec that this block of pagination params appears about 25 times (with slight variations around sort mode). Obviously improving the human-readability and length of a machine-oriented spec is not particularly urgent, but it looks like this could be shortened quite a bit with global components.

"parameters": [
  {
    "in": "query",
    "name": "limit",
    "description": "Maximum number of items returned by a single call",
    "schema": {
      "nullable": true,
      "type": "integer",
      "format": "uint32",
      "minimum": 1
    },
    "style": "form"
  },
  {
    "in": "query",
    "name": "page_token",
    "description": "Token returned by previous call to retrieve the subsequent page",
    "schema": {
      "nullable": true,
      "type": "string"
    },
    "style": "form"
  },
  {
    "in": "query",
    "name": "sort_by",
    "schema": {
      "$ref": "#/components/schemas/IdSortMode"
    },
    "style": "form"
  }
],

Start with the Omicron OpenAPI spec at openapi/nexus.json#L20-L51 and compare the repeated pagination parameter blocks, including the sort-mode variations. Trace how Dropshot generates this spec, then move common parameters into OpenAPI components and replace repeated definitions with references. Done means the generated spec is shorter, preserves the variations, and remains valid.