AML: Allow calling online endpoints directly with a payload

**Is your feature request related to a problem? Please describe.**
It is very frustrating having to dump payloads to a temporary file each time we want to invoke an online endpoint because the api does not support directly supplying the payload

**Describe the solution you'd like**
Implement the possibility of directly passing payloads to the invoke method

**Describe alternatives you've considered**
Using a tempfile for dumping payloads only to be directly read into memory again by the api.

**Additional context**
It is clear that this was originally intended to be implemented because the function already defines an `input_data` variable and disables pylint from detecting it is unused:
https://github.com/Azure/azure-sdk-for-python/blob/c33e4f711b82c283b60fbafd292474d5ca6ce393/sdk/ml/azure-ai-ml/azure/ai/ml/operations/_online_endpoint_operations.py#L307-L308

Implementing this should be as simple as:
```python
if input_data and request_file:
raise ValueError("Only one of input_data or request_file can be specified, not both!")

if input_data:
data = input_data
# Alternatively perform a little more validating here if you prefer.
else:
with open(request_file, "rb") as f:
data = json.loads(f.read())

... continue with rest of existing code
```

@vebjornmalmin @kribra