Using KFP requires a setting of DOCLING_SERVE_ENG_KFP_EXPERIMENTAL

It seems when using KFP, the setting of DOCLING_SERVE_ENG_KFP_EXPERIMENTAL is needed.

However, there is no way to set this in the Doclingserve CR.

To reproduce deploy the Docling Operator and create a Doclingserve instance like this:
```
apiVersion: docling.github.io/v1alpha1
kind: DoclingServe
metadata:
labels:
app.kubernetes.io/name: docling-operator
app.kubernetes.io/managed-by: kustomize
name: docling-server
namespace: docling
spec:
apiServer:
image: "quay.io/docling-project/docling-serve:v0.13.0"
enableUI: true
instances: 1
engine:
kfp:
endpoint: "https://ds-pipeline-dspa.docling.svc.cluster.local:8888"
```
Observe the following error in the Doclingserve pod
```
Traceback (most recent call last):
File "/opt/app-root/bin/docling-serve", line 4, in
from docling_serve.__main__ import main
File "/opt/app-root/src/docling_serve/__main__.py", line 13, in
from docling_serve.settings import docling_serve_settings, uvicorn_settings
File "/opt/app-root/src/docling_serve/settings.py", line 88, in
docling_serve_settings = DoclingServeSettings()
^^^^^^^^^^^^^^^^^^^^^^
File "/opt/app-root/lib64/python3.12/site-packages/pydantic_settings/main.py", line 176, in __init__
super().__init__(
File "/opt/app-root/lib64/python3.12/site-packages/pydantic/main.py", line 214, in __init__
validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for DoclingServeSettings
Value error, KFP is not yet working. To enable the development version, you must set DOCLING_SERVE_ENG_KFP_EXPERIMENTAL=true. [type=value_error, input_value={'enable_ui': 'true', 'ar...svc.cluster.local:8888'}, input_type=dict]
For further information visit https://errors.pydantic.dev/2.10/v/value_error
```