[Feature Request]: eagerly evaluating pipeline starting with Create() in Python

### What would you like to happen?

For documentation and testing, I find it super convenient to be able to pass Python objects such as a list into a PTransform, which causes the PTransform to be evaluated eagerly, e.g.,
```
>>> [1, 2, 3] | beam.Map(lambda x: x ** 2)
[1, 4, 9]
```

I would like to be able to do the same sort of thing for pipelines that start with `beam.Create()` rather than an existing PCollection. This would be particularly helpful for examples where I use `beam.Create()` inside my library's own custom PTransforms (and thus cannot simply unwrapped `beam.Create`).

This would need to use some sort of special placeholder value, e.g., perhaps `None` or another sentinel value/class, e.g., perhaps
```
>>> beam.EagerPipeline() | beam.Create([1, 2, 3]) | beam.Map(lambda x: x ** 2)
[1, 4, 9]
```

To solve this problem inside Xarray-Beam (for unit-testing), I wrote a funny EagerPipeline class that actually serializes the results to a temporary file on disk:
https://github.com/google/xarray-beam/blob/037d1f3ea077a9d97e614dd4ac2959ee45633df9/xarray_beam/_src/test_util.py#L32-L51

I guess I could externalize this to my users but I would love to be able to point to something upstream.

### Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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