Add InterfaceCustomUnmarshaler and test case to override interface type

[ozraru]

Close #449

1. Add CustomInterfaceMarshaler and CustomInterfaceUnmarshaler.

There are Bytes(Un)Marshaler and Interface(Un)Marshaler interface.
But there is no Interface(Un)Marshaler in Custom(Un)Marshaler.

I want to use InterfaceUnmashaler as CustomUnmarshaler.

Renaming Custom(Un)marshaler to CustomBytes(Un)marshaler is good for describing function, but it breaks backward compatibility.

The linked issue has a use case and example.

2. Add test cases to override interface type

Added test case corresponding to the example in the linked issue.

• Describe the purpose for which you created this PR.
• Create test code that corresponds to the modification

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.91597% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 76.26%. Comparing base (4653a1b) to head (fe1b2f9).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
+ Coverage   76.02%   76.26%   +0.24%     
==========================================
  Files          13       13              
  Lines        4692     4790      +98     
==========================================
+ Hits         3567     3653      +86     
- Misses        866      875       +9     
- Partials      259      262       +3