Add support for HTTP Server Sent Events (SSE) in controller generation

Nobody has claimed this yet.

### Feature description

# Description

The generator should add support for HTTP Server Sent Events using Micronaut's [`Event`](https://docs.micronaut.io/latest/guide/index.html#sse) API.

Given a valid OpenAPI schema, if a `text/event-stream` content type is encountered in a response definition during generation, the generator should:
1. Select the relevant output schema (as currently done for normal request-response content types) (🍐)
2. Generate a corresponding type `T`
3. Set `Publisher>>` as the return type of the generated controller method
4. Append `text/event-stream` to the list of content types inside the HTTP method annotation or via a `@Produces` annotation

(🍐) I don't know `micronaut-openapi`'s behavior in detail, but I suppose the library selects the schema associated to 200 HTTP status code.

## Additional notes

- The return type should be wrapped in a `Mono` or the method marked `suspend` if the corresponding generator options are enabled (consistent with the current generator behavior)
- Multiple HTTP response codes returning a `text/event-stream` should not have explicit support
- By default, we could also add a `@ExecuteOn(TaskExecutors.IO)` annotation and potentially make the executor controllable per-method via an extension (e.g. `x-executor: blocking|io|message_consumer|scheduled|virtual`, list from [here](https://docs.micronaut.io/latest/api/io/micronaut/scheduling/TaskExecutors.html))

## Example OpenAPI schema

```yaml
openapi: 3.0.1
info:
title: Unik
description: Unik API
version: "0.0"
paths:
/users/stream:
get:
operationId: streamNewUsers
responses:
200:
description: Successful response
content:
text/event-stream:
schema:
$ref: "#/components/schemas/User"
409:
description: Can't stream users
content:
application/json:
schema:
$ref: "#/components/schemas/ApiProblem"
components:
schemas:
User:
type: object
ApiProblem:
type: object
```

## Expected generated controller interface

```kotlin
import com.generated.api.User
import io.micronaut.http.sse.Event
import org.reactivestreams.Publisher

/* -- snip -- */

interface DefaultApi {
@Get("/users/stream")
@Produces("text/event-stream", "application/json")
suspend fun streamNewUsers(): Publisher>
}
```

Start by tracing the controller-generation path that handles response content types and the existing normal request-response schema selection. Compare its output with the Kotlin example in the issue. Done means a text/event-stream response generates Publisher>, preserves the relevant schema, adds the stream content type, and follows the existing Mono or suspend options.

Written by the indexing model from the issue text.