Clean-up string usage

## Brief feature description
Iceoryx uses a variety of strings (`std::string`, `cxx::string<>`, `cxx::CString100`, etc).
Once #253 is merged this mishmash should be cleaned-up.

## Detailed information
Only use `cxx:string<>` or types created from it.

- [x] No std::string in public API

- [ ] No more std::strings in code base
- [ ] iceoryx_hoofs
- [x] Taken from: #1196 Refactor `cxx::convert` so that all `cxx::string` capacities are handled
- [ ] iceoryx_posh
- [x] ~~iceoryx_dds~~

- [x] No more CSring100 in code base

- [ ] Common convention for string aliases. Currently we have:
- alias without `_t`, e.g. `IdString`
- alias with `_t`, e.g. `ProcessName_t`
- alias nested in a class, e.g. `PosixUser::string_t`
- alias with a constexpr as string length, e.g. `cxx::string`
- alias without a constexpr as string length, e.g. `cxx::string<100>`
- see also https://github.com/eclipse/iceoryx/pull/373/files#r525922015 and https://github.com/eclipse/iceoryx/pull/373/files#r525922656
- [ ] cleanup iceoryx_hoofs
- [x] cleanup iceoryx_posh
- [x] cleanup iceoryx_dds