Add Sample.Any to the Python SDK to match Java's Sample.any

[SreeramaYeshwanthGowd]

Problem

Java has Sample.any(n), which returns up to n arbitrary elements from a PCollection without the cost of a uniform random sample. The Python SDK has no equivalent. Its only global option is Sample.FixedSizeGlobally(n), which runs a uniform reservoir sample and returns a single list[T]. This gap was reported in #18552.

Solution

Add Sample.Any, the Python equivalent of Java's Sample.any. It keeps up to n arbitrary elements using a small CombineFn and returns them as a PCollection of individual elements. It makes no uniformity guarantee and is cheaper than FixedSizeGlobally because it skips the per element random sampling. If the input has fewer than n elements, all of them are returned. This mirrors Java's semantics and its element returning result type.

Testing

Added unit tests in combiners_test.py that run on the DirectRunner and cover the normal case, an input smaller than n, a windowed input, an empty input, and n equal to zero. Also extended the existing display data test to cover the new transform.

Fixes #18552

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