Move Flatmap implementation to `iceoryx2` crate, keep pure Rust version in `iceoryx2-bb-container`

[FerdinandSpitzschnueffler]

For the C language binding of the blackboard, the Flatmap API was adapted in a "C-style" way so that a key compare function, expecting u8 pointers, can be passed. As this is very tailored to the blackboard usage, it would be better to move the flatmap implementation into the iceoryx2 crate and keep a pure and idiomatic Rust version in iceoryx2-bb-container.