[C++][Pistache] Add comparison operators

[muttleyxd]

Add comparison operators for generated models in struct model

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@ravinikam @stkrwork @etherealjoy @MartinDelille @muttleyxd

[wing328]

@muttleyxd thanks for the PR, which has been included in the v4.2.2 release: https://twitter.com/oas_generator/status/1201432648544972800