Limit input to PipelineParameter

It would be nice if you could limit the possible inpute to a pipeline parameter.
E.g. if you want to parameterize the metric an AutoML run uses as primary metric it could look like this:

```python
METRIC_NAMES = ['mean_absolute_percentage_error', 'normalized_root_mean_squared_error', 'root_mean_squared_log_error', 'r2_score', 'spearman_correlation', 'median_absolute_error', 'root_mean_squared_error', 'normalized_mean_absolute_error', 'mean_absolute_error', 'normalized_root_mean_squared_log_error', 'explained_variance', 'normalized_median_absolute_error']

metric = PipelineParameter(name="metric", default_value="r2_score", limit_input_to=METRIC_NAMES)
```

In the UI this could then be a dropdown similar to available Datasets if you specify a Dataset PipelineParameter. When submitting with the SDK it should throw an error and display the possible inputs.

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#### Document Details

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* ID: 8e3ec7f7-25c2-8f63-331c-2eb62ffb73c7
* Version Independent ID: 4e31dffb-12fd-85d9-a1a2-aa038017d075
* Content: [azureml.pipeline.core.graph.PipelineParameter class - Azure Machine Learning Python](https://docs.microsoft.com/en-us/python/api/azureml-pipeline-core/azureml.pipeline.core.graph.pipelineparameter?view=azure-ml-py)
* Content Source: [AzureML-Docset/stable/docs-ref-autogen/azureml-pipeline-core/azureml.pipeline.core.graph.PipelineParameter.yml](https://github.com/MicrosoftDocs/MachineLearning-Python-pr/blob/live/AzureML-Docset/stable/docs-ref-autogen/azureml-pipeline-core/azureml.pipeline.core.graph.PipelineParameter.yml)
* Service: **machine-learning**
* Sub-service: **core**
* GitHub Login: @DebFro
* Microsoft Alias: **debfro**