ffi: Extract some serialization methods for use in other files.

[LinZhihao-723]

Description

In the current IR stream implementation, serialization methods are made as local methods so they cannot be reused in other files. However, some methods are reuseable when we implement the next IR format. This PR moves some of the general methods to util files to make them reusable. Methods moved including:

• serialie_metadata: This general method serializes a JSON object as the IR stream metadata. It can be reused when creating a new format IR stream.
• serialize_int: This general integer sterilization method will be wildly used in integer value encoding in the coming format.

Notice that minor refactoring is also introduced to meet our latest coding standard when moving these methods.

Validation performed

Ensured the code can be successfully built with all unit tests passed.

[kirkrodrigues]

For the PR title, how about:

ffi: Extract some serialization methods for use in other files.

[LinZhihao-723]

For the PR title, how about:

ffi: Extract some serialization methods for use in other files.

lgtm