YT-CPPHGL-26: Implement flat list hypergraph representation models

[SpectraL519]

• Defined a new hypergraph implementation tag: flat_list_t<LayoutTag>
• Aligned existing and added new implementation-tag-related type traits
• Created a new internal implementation class: flat_incidence_list with specializations for each layout tag
• Moved header files from the hgl/types/ directory to hgl/
• Extended the segmented_vector class with the following methods:
  • Data getters: data_view() (previously data()), data_storage(), data_ptr()
  • Offsets getters: offsets_view(), offsets_storage(), offsets_ptr()
  • Other: empty(seg)

[SpectraL519]

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