Make _PipelineExecution public

[navaj0]

Describe the feature you'd like

_PipelineExecution is a handy class that allow users to view and control executions, even if they are not started by the Python SDK. For example, I have an execution "blah" started from the Studio UI

execution = _PipelineExecution(arn="blah")
execution.result()

However, it is marked as protected.

Make it a public class and add documentation with code examples.

How would this feature be used? Please describe.

execution = PipelineExecution(arn="blah")
execution.result()

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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