OpenAPI 3.1 Support

Hi I think Goa would profit from OpenAPI 3.1 support.

Therefore I create this PR initially implementing it: https://github.com/goadesign/goa/pull/3755

This issue is to discuss it in a more formal way so we can shape and refine it till it is mergeable (if desired ofc).

I tried to do it in a way that:

- Respects Goa's existing patterns and conventions
- Provides a clean, intuitive API for developers
- Properly leverages OpenAPI 3.1's webhook support
- Maintains backward compatibility (webhooks are additive, not breaking)

```
API Service Type ResultType
├── Title ├── Description ├── Extend ├── TypeName
├── Description ├── Docs ├── Reference ├── ContentType
├── Version ├── Security ├── ConvertTo ├── Extend
├── Docs ├── Error ├── CreateFrom ├── Reference
├── License ├── GRPC ├── Attribute ├── ConvertTo
├── TermsOfService ├── HTTP ├── Field ├── CreateFrom
├── Contact ├── Method └── Required ├── Attributes
├── Server │ ├── Payload └── View
└── HTTP │ ├── Result
│ ├── Error
│ ├── GRPC
│ └── HTTP
├── Files
└── Webhook
├── WebhookDescription
├── WebhookPayload
└── WebhookHTTP
├── WebhookMethod/POST/GET/etc.
├── WebhookPath
├── WebhookHeaders
└── WebhookResponse

```