* get data field with view
* refine tc
* refine based on flake check
* run black again
* rebase upstream master
* add comment to tc
* refine raise exception
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 .
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Co-authored-by: Brian Xu <brian.xu1@bytedance.com>
