Add skip attribute to serialization proc macros

[Lorak-mmk]

As requested in #899, this PR adds a field attribute to SerializeCql and SerializeRow macros that allow to skip given field during serialization.
This can be useful for fields unrelated to database.

Fixes: #899

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
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• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
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