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>

examples/py_custom_message_builder.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+
+import capnp  # noqa: F401
+import addressbook_capnp
+
+
+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)
+
+
+person = addressbook_capnp.Person.new_message(allocate_seg_callable=Allocator())
+
+person.init("extraData", 5)
+print(person.extraData)
+print(bytes(person.extraData))
+print(type(person.extraData))
+print()
+
+person.extraData[1] = 0xFF
+print(person.extraData)
+print(bytes(person.extraData))
+print()
+
+person.extraData = b"hello"
+print(person.extraData)
+print(bytes(person.extraData))
+print(type(person.extraData))
+print()
+
+person = person.as_reader()
+print(person.extraData)
+print(bytes(person.extraData))
+print(type(person.extraData))