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>

capnp/includes/PyCustomMessageBuilder.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "Python.h"
+#include <capnp/message.h>
+#include <capnp/serialize.h>
+#include <vector>
+
+namespace capnp {
+
+class PyCustomMessageBuilder : public capnp::MessageBuilder {
+public:
+  explicit PyCustomMessageBuilder(PyObject* allocateSegmentCallable,
+  uint firstSegmentWords = capnp::SUGGESTED_FIRST_SEGMENT_WORDS);
+
+  ~PyCustomMessageBuilder() noexcept(false) override;
+
+  kj::ArrayPtr<capnp::word> allocateSegment(capnp::uint minimumSize) override;
+
+private:
+  PyObject* allocateSegmentCallable;
+
+  uint firstSize;
+  uint curSize = 0;
+
+  std::vector<PyObject*> allocatedBuffers;
+};
+
+}