Provide a CRT-native async HeadObject

### Describe the feature

On `S3CrtClient`, `GetObject`/`PutObject`/`CopyObject` are all dispatched as CRT meta-requests (`aws_s3_client_make_meta_request`) and complete on a CRT event-loop thread. `HeadObject` is not: the synchronous method uses the blocking `InvokeServiceOperation` HTTP path, and the generated `HeadObjectAsync` is just a thin `SubmitAsync(&S3CrtClient::HeadObject, ...)` wrapper that runs that blocking call on the SDK thread executor (parking one executor thread per in-flight HEAD).

Please add a CRT-native `HeadObjectAsync` (event-loop dispatch, completion on an event-loop thread) alongside the other three.

### Use Case

Workloads that gate a write on object existence (Head-before-PUT) currently can't get the HEAD off the blocking path. When every other operation on the hot path is async on the event loop, the synchronous HEAD becomes the bottleneck: concurrency is capped by the SDK executor pool size and one executor thread is parked per in-flight HEAD, rather than being bounded by the caller's own in-flight limit like the async GET/PUT. A CRT-native `HeadObjectAsync` lets existence checks scale with the same concurrency budget as the rest of the I/O.

### Proposed Solution

`aws_s3_clint_make_meta_request` already issues arbitrary single requests on the event loop via `AWS_S3_META_REQUEST_TYPE_DEFUALT` with `operation_name` set (that is exactly how `CopyObjectAsync` is implemented today) and `"HeadObject"` is a registered canonical operation name.

**Implementation sketch:**
Mirror `CopyObjectAsync` (the body-less DEFAULT meta-request sibling), with:
- `options.type = AWS_S3_META_REQUEST_TYPE_DEFAULT`
- `options.operation_name = "HeadObject"`
- HTTP method `HEAD`
- A shutdown callback that builds `HeadObjectResult` purely from response headers. `HEAD` has no body.
- A HEAD-specific finish callback that preserves the body-less HTTP-status → error mapping. See below.

### Other Information

**One correctness subtlety:**

A correct HeadObjectAsync must preserve the body-less HTTP-status → error mapping — e.g. a HEAD-specific finish callback that records the response status and any error headers but does not set a client-error type when an HTTP response was received, leaving the client-error type for the transport / no-response case only.

**Related (pre-existing): `userData` leak when `make_meta_request` returns null**

Independent of HeadObject, the existing async Get/Put/Copy paths leak their `CrtRequestCallbackUserData` (allocated with `Aws::New`) when `aws_s3_client_make_meta_request` returns null: that branch returns via the handler without the shutdown callback ever running, and the shutdown callback is the only place `Aws::Delete(userData)` is called (see `CopyObjectAsync` / `GetObjectAsync` / `PutObjectAsync`). It only triggers on meta-request creation failure (effectively allocation failure), so the practical impact is negligible — but any new `HeadObjectAsync` should avoid reproducing it (free `userData` on the null-meta-request branch), and the existing methods could be fixed in passing.

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

Beginne mit dem Lesen der Implementierungen von S3CrtClient's CopyObjectAsync, GetObjectAsync und PutObjectAsync und verfolge anschließend aws_s3_client_make_meta_request sowie deren Shutdown- und Finish-Callbacks. Implementiere den Event-Loop-Pfad für HeadObject mit der angegebenen DEFAULT-Operation, der HEAD-Methode, der Verarbeitung eines ausschließlich Header enthaltenden Ergebnisses, der Statuszuordnung und der Bereinigung von Null-Requests; fertig ist die Aufgabe, wenn die asynchrone Anfrage den blockierenden Executor-Pfad vermeidet und Transportfehler von HTTP-Fehlern unterscheidet.

Vom Indexierungsmodell aus dem Issue-Text verfasst.