Add equal_to_approx matcher for approximate numeric assertions

[SreeramaYeshwanthGowd]

Problem

The assert_that matchers only offer exact equality through equal_to.
Pipelines that produce floating point results are awkward to assert, because
callers have to hand write an equals_fn in every test. Issue #18028 asks to
expand the set of matchers.

Solution

Add equal_to_approx, an assert_that matcher that behaves like equal_to for
ordering and membership but compares real number elements, including numbers
nested in tuples or lists, with math.isclose using configurable rel_tol and
abs_tol. All other elements are compared with ==. It is a thin wrapper over
the existing equal_to(equals_fn=...) mechanism and adds no new dependency.

Testing

Added unit tests in util_test.py covering values within and outside tolerance
(including a nested value outside tolerance), nested tuples and lists, rel_tol
and abs_tol, non numeric passthrough, and an empty PCollection.

Fixes #18028

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