Fix #1067: Add basic support for examples as well as example

[jceipek]

This PR fixes the following bug, #1067: the examples keyword is not supported at all in some contexts, which causes some response fields to be shown with a generic example even though a schema-supplied example is available.

In cases where it wasn't supported at all, we instead now pick the first example from examples, which is a valuable incremental improvement and matches the behavior of https://editor-next.swagger.io/.

Given the following minimal, valid spec:

{
  "openapi": "3.1.0",
  "info": {
    "title": "Test API",
    "version": "0.0.1",
    "description": "Minimal test"
  },
  "paths": {
    "/api/test": {
      "post": {
        "parameters": [],
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "application/json": {
                "schema": {
                  "properties": {
                    "list_field": {
                      "description": "List of Strings",
                      "examples": [["Bug: missing from output"]],
                      "items": { "type": "string" },
                      "type": "array"
                    },
                    "list_field_single_example": {
                      "description": "List of Strings single example",
                      "example": ["Deprecated but included in output"],
                      "items": { "type": "string" },
                      "type": "array"
                    },
                    "string_field": {
                      "description": "String Field",
                      "examples": ["Included in output", "Missing from output since only one example is displayed"],
                      "type": "string"
                    }
                  },
                  "required": [
                    "list_field",
                    "string_field"
                  ],
                  "title": "Demo",
                  "type": "object"
                }
              }
            }
          }
        }
      }
    }
  }
}

Before this PR, the example response was:

{
  "list_field": [
    "string"
  ],
  "list_field_single_example": [
    "Deprecated but included in output"
  ],
  "string_field": "Included in output"
}

Note [ "string" ] under list_field.

Now the example response is:

{
  "list_field": [
    "Bug: missing from output"
  ],
  "list_field_single_example": [
    "Deprecated but included in output"
  ],
  "string_field": "Included in output"
}

Note [ "Bug: missing from output" ] under list_field.