Add .copy_with(**kwargs) convenience method to manifests (#242)

[deekshaNVIDIA]

Summary

Closes #242.

Lhotse treats manifest objects as (effectively) immutable and uses the fastcopy(obj, field=...) idiom all over the codebase to produce a modified copy. As suggested in #242, this adds a .copy_with(**kwargs) member method so users can do the same without importing fastcopy, in a way that reads naturally and composes nicely with comprehensions:

sup2 = supervision.copy_with(text="new text")
cut2 = cut.copy_with(start=5.0)
rec2 = recording.copy_with(id="new-id")

What changed

copy_with(**kwargs) was added to the core manifest types:

• CustomFieldMixin (lhotse/custom.py) — covers SupervisionSegment, DataCut/MonoCut/MultiCut, and the text examples.
• Cut base class (lhotse/cut/base.py) — covers PaddingCut and MixedCut; delegates to the existing Cut.copy() so cuts expose the same name as every other manifest (this is exactly the cut.copy_with(start=5.0) example from the issue).
• Recording, Features, Array, TemporalArray — thin wrappers around fastcopy.

All implementations delegate to fastcopy, so behavior/semantics are identical to the existing idiom.

Tests

test/test_copy_with.py parametrizes over every manifest type and verifies that copy_with:

• overwrites the requested field,
• returns a new object and does not mutate the original,
• matches fastcopy(obj, **kwargs),
• is a no-op-equivalent when called with no kwargs,
• and can set custom fields on SupervisionSegment.

37 passed locally. I also ran test/test_supervision_set.py, test/cut/test_padding_cut.py, and test/features/test_temporal_array.py with no regressions (one unrelated failure, test_serialize_padded_cut_set, is a Windows-only NamedTemporaryFile reopen quirk). black, isort --profile black, and flake8 --select=E9,F63,F7,F82 are clean on the changed files.