pycapnp supports lazy zero segment allocation strategy

[BrianXu0623]

Summary

This PR is for resolving the following issue:
Skip zeroing for guaranteed-overwritten data in SegmentAllocator #2400

This PR adds support for lazy zero segement allocation strategy to pycapnp.

• Introduces BuilderOptions (lazyZeroSegment, skipZeroData).
• MessageBuilder and struct new_message APIs now accept these options.
• Adds an example (examples/lazy_zero_allocation.py) to demonstrate usage.

This allows users to fine-tune memory allocation behavior for performance or special environments.
No breaking changes; default behavior is unchanged.

Example Usage

class Allocator:
    def __init__(self):
        return

    def __call__(self, minimum_size: int) -> bytearray:
        # dirty the memory on purpose, to simulate reusing shared memory
        return bytearray(random.getrandbits(8) for _ in range(minimum_size * WORD_SIZE))


# Set lazyZeroSegment and skipZeroData to True
lazy_zero = capnp.LazyZeroSegmentAlloc(enableLazyZero=True, skipLazyZeroTypes={capnp.types.Data})
builder_options=capnp.BuilderOptions(lazyZeroSegmentAlloc=lazy_zero)
# Create a new Person message with custom allocation options
person = addressbook_capnp.Person.new_message(allocate_seg_callable=Allocator(), builder_options=builder_options)
print(person.name) # guaranteed empty string "" 
person.init("extraData", 100)
print(bytes(person.extraData)) # random bytes