Redesign the way processors are identified; fixes #653

[liuzicheng1987]

No description provided.

[gemini-code-assist]

Code Review

This pull request refactors the library's processor logic by migrating static configuration flags from the Processors struct into standalone type traits within the rfl::internal namespace. This change affects multiple format parsers, including Avro, BSON, JSON, and Msgpack, which now utilize traits like no_field_names_v and default_if_missing_v. The review feedback identifies several issues in the newly added internal headers, such as missing includes for Processors.hpp, inconsistent namespace declarations, outdated header guards, and redundant forward declarations.