Ingress additionalIngresses backend service name is hardcoded, cannot be overridden

## Description
When using `hdx-oss-v2.ingress.additionalIngresses` to expose the OpenTelemetry Collector, the generated Ingress `backend.service.name` is **hardcoded** to the chart’s default fullname (e.g., `clickstack-hdx-oss-v2`).
If the actual Service name differs (e.g., `clickstack-hdx-oss-v2-otel-collector`), the Ingress points to a non‑existent Service and returns:

```
503 Service Temporarily Unavailable
```

---

## To Reproduce
1. Deploy the chart with `additionalIngresses` configured for OTLP/HTTP and OTLP/gRPC.
2. The collector Service is created as:
```
clickstack-hdx-oss-v2-otel-collector
```
with ports `4317` and `4318`.
3. Run:
```bash
kubectl describe ingress -n clickstack
```
4. Output shows:
```
/ clickstack-hdx-oss-v2:4318 ()
```
5. Requests to the ingress host return `503`.

---

## Expected behavior
The Ingress backend should:
- Point to the actual collector Service name, **or**
- Allow overriding the backend service name in `values.yaml`.

---

## Actual behavior
The backend service name is fixed to the chart’s default fullname, ignoring the actual Service name used for the collector.

---

## Proposed solution
Add a `serviceName` (or `service.name`) field under each `additionalIngresses.hosts.paths` entry in `values.yaml` that, if set, overrides the default fullname in the Ingress template.

**Example values.yaml:**
```yaml
additionalIngresses:
- name: otel-collector-http
hosts:
- host: otel-http.example.com
paths:
- path: /
pathType: Prefix
serviceName: clickstack-hdx-oss-v2-otel-collector
port: 4318
```

**Ingress template change:**
```yaml
service:
name: {{ .path.serviceName | default (include "hdx-oss-v2.fullname" $) }}
port:
number: {{ .path.port }}
```

---

## Environment
- **Chart version:** `0.7.2`
- **Kubernetes version:** `v1.33.4`
- **Ingress controller:** NGINX

---

## Additional context
This limitation makes it impossible to expose the collector via `additionalIngresses` unless the collector’s Service name matches the chart’s default fullname exactly.
Allowing a service name override would make the feature usable in more setups.