Support Webhook operationId output when explicitly passed

[djm]

Prior to this commit, providing operation_id to extend_schema on a webhook object would not have any effect on the resulting schema.

The OpenAPI 3.1 spec added support for webhooks being their own top-level definition. Each Webhook is an Operation Object.

Operation Objects are defined in the spec @ 4.8.10, and it dictates that they may contain an operationedId string field:

Unique string used to identify the operation. The id MUST be unique among
all operations described in the API. The operationId value is case-sensitive.
Tools and libraries MAY use the operationId to uniquely identify an operation,
therefore, it is RECOMMENDED to follow common programming naming conventions.

Some platforms, such as Fern docs, require operationId on webhooks in the schema to identify them. Without the ability to output operationId users cannot use their schema with services like this.

We took the decision to only output explicitly provided operationIds for two reasons:

1. operationIds are not marked as REQUIRED by the spec in the languge.
2. This will allow users to opt-in to this feature rather than have their webhooks defined with auto-generated names. The auto-name generation relies on the path segment which webhooks don't have and so users would have uniqueness issues here unless we also changed that algorithm.