Option to control behaviour of `allOf`?

Start by tracing the allOf and anyOf generation paths that produce PullRequestClosedPullRequest, then review the PullRequestEvent extraction boilerplate described in the issue. Compare the generated types with the linked schema and define a concrete configurable behavior with tests showing how consumers access shared pull_request fields; done requires an agreed API and regression coverage.

Consider the following schema:

https://unpkg.com/@octokit/webhooks-schemas@7.3.1/schema.json

it contains a definition:

    "pull_request$closed": {
      "$schema": "http://json-schema.org/draft-07/schema",
      "type": "object",
      "required": ["action", "number", "pull_request", "repository", "sender"],
      "properties": {
        "action": { "type": "string", "enum": ["closed"] },
        "number": {
          "type": "integer",
          "description": "The pull request number."
        },
        "pull_request": {
          "allOf": [
            { "$ref": "#/definitions/pull-request" },
            {
              "type": "object",
              "required": ["state", "closed_at", "merged"],
              "properties": {
                "state": {
                  "type": "string",
                  "enum": ["closed"],
                  "description": "State of this Pull Request. Either `open` or `closed`."
                },
                "closed_at": { "type": "string", "format": "date-time" },
                "merged": { "type": "boolean" }
              },
              "tsAdditionalProperties": false
            }
          ]
        },
        "repository": { "$ref": "#/definitions/repository" },
        "installation": { "$ref": "#/definitions/installation-lite" },
        "organization": { "$ref": "#/definitions/organization" },
        "sender": { "$ref": "#/definitions/user" }
      },
      "additionalProperties": false,
      "title": "pull_request closed event"
    },

allOf inside pull_request would result of generation of type PullRequestClosedPullRequest which is almost the same as normal PullRequest but has one extra field. This creates a problem for consuming those types: let's say you need a field from inside pull_request -- you would have to account for all of the variations of it.

In languages like TS this would be solved by structural subtyping; in rust, while there's nothing like that built-in, perhaps there's a way to adjust behaviour of anyOf to generate some code that would be able to return common parts? e. g. via a generated trait for example. Right now this needs a lot of boilerplate code, for example to extract any pull_request field form PullRequestEvent