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_blob_to_dict_base64.py

@@ -1,6 +1,5 @@
 import os
 import capnp
-import base64
 import pytest
 
 this_dir = os.path.dirname(__file__)
@@ -15,7 +14,7 @@ def test_blob_to_dict(blob_schema):
     blob_value = b"hello world"
     blob = blob_schema.BlobTest(blob=blob_value)
     blob_dict = blob.to_dict(encode_bytes_as_base64=True)
-    assert base64.b64decode(blob_dict["blob"]) == blob_value
+    assert blob_dict["blob"].tobytes() == blob_value
     msg = blob_schema.BlobTest.new_message()
     msg.from_dict(blob_dict)
     assert blob.blob == blob_value