Support python custom message builder and make Data field's type return MemoryView (#380)

This PR is for resolving the following issue:
[issue](https://github.com/capnproto/pycapnp/issues/379)

1. Created `_PyCustomMessageBuilder` extends `MessageBuilder`, enabling the ability to customise the `SegmentAllocate` method in Python. This allows allocation and data population within shared memory, and supports zero-copy inter-process data transfer by passing segment offsets.

2. Fields of type `Data` now support being set with a `memoryview`. When retrieving a `Data` field from a `DynamicStructBuilder`, it will return a writable `memoryview`, allowing users to modify the data directly. This enables memory to be pre-allocated and content to be modified in later, eliminating an extra copy. When retrieving a `Data` field from a `DynamicStructReader`, it will return a read-only `memoryview`, allowing user to read data without memory copy.

* add memoryview and custom builder

* support set dynamic field

* add curSize

* add initialSize and lastSize

* change StringPtr name

* add test case

* refine test case

* convert func to py callable object

* add initial value

* refine example

* add copy as_reader and new_message, make structReader's data field return RO memoryView

* rebase master and bugfix

* reformat flake8

* refine test case

* refine test cases for blob

* remove unused import for flake8

* run black .

---------

Co-authored-by: Brian Xu <brian.xu1@bytedance.com>

test/test_py_custom_message_builder.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python3
+
+import pytest
+import capnp  # noqa: F401
+import os
+
+this_dir = os.path.dirname(__file__)
+
+
+@pytest.fixture
+def all_types():
+    return capnp.load(os.path.join(this_dir, "all_types.capnp"))
+
+
+def test_addressbook(all_types):
+    class Allocator:
+        def __init__(self):
+            self.cur_size = 0
+            self.last_size = 0
+
+        def __call__(self, minimum_size: int) -> bytearray:
+            actual_size = max(minimum_size, self.cur_size)
+            print(
+                f"minimum_size: {minimum_size}, last_size: {self.last_size}, "
+                f"actual_size: {actual_size}, cur_size: {self.cur_size}"
+            )
+            self.last_size = actual_size
+            self.cur_size += actual_size
+            WORD_SIZE = 8
+            byte_count = actual_size * WORD_SIZE
+            return bytearray(byte_count)
+
+    allocator = Allocator()
+    assert allocator.cur_size == 0
+    assert allocator.last_size == 0
+    msg_builder = capnp._PyCustomMessageBuilder(allocator, 1024)
+    struct_builder = msg_builder.init_root(all_types.TestAllTypes)
+    assert allocator.cur_size == 1024
+    assert allocator.last_size == 1024
+
+    struct_builder.init("dataField", 5)
+    assert struct_builder._get("dataField") == b"\x00\x00\x00\x00\x00"
+
+    struct_builder._get("dataField")[1] = 0xFF
+    assert struct_builder._get("dataField") == b"\x00\xff\x00\x00\x00"
+
+    struct_builder.dataField = b"hello"
+    assert struct_builder._get("dataField") == b"hello"
+
+    struct_builder = struct_builder.as_reader()
+    assert struct_builder._get("dataField") == b"hello"