Add external dependencies option

[uilianries]

Add support to use external dependencies instead of vendorized ones. The new CMake option USE_SYSTEM_DEPENDENCIES, uses find_package to include (mandatory) fmt and gtest. In case that option is enabled, and the compiler has std::format supported, CMake will use the external dependencies, because the user configured it explicitly.

Added a new CI job to validate the configuration.

closes #546

[cieslarmichal]

Nice

[uilianries]

@cieslarmichal The current error is due -Werror with unused variable:

FAILED: CMakeFiles/faker-cxx.dir/src/modules/string/String.cpp.o 
/usr/bin/g++-13  -I/home/runner/work/faker-cxx/faker-cxx/include -isystem /home/runner/.conan2/p/b/fmt808a6edee61f6/p/include -m64 -O3 -DNDEBUG -std=c++20 -Wall -Wextra -Wpedantic -Wconversion -Wformat -Werror -MD -MT CMakeFiles/faker-cxx.dir/src/modules/string/String.cpp.o -MF CMakeFiles/faker-cxx.dir/src/modules/string/String.cpp.o.d -o CMakeFiles/faker-cxx.dir/src/modules/string/String.cpp.o -c /home/runner/work/faker-cxx/faker-cxx/src/modules/string/String.cpp
In file included from /usr/include/c++/13/map:62,
                 from /home/runner/work/faker-cxx/faker-cxx/include/faker-cxx/String.h:4,
                 from /home/runner/work/faker-cxx/faker-cxx/src/modules/string/String.cpp:1:
In member function ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_lower_bound(_Link_type, _Base_ptr, const _Key&) [with _Key = char; _Val = std::pair<const char, faker::CharCount>; _KeyOfValue = std::_Select1st<std::pair<const char, faker::CharCount> >; _Compare = std::less<char>; _Alloc = std::allocator<std::pair<const char, faker::CharCount> >]’,
    inlined from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::find(const _Key&) [with _Key = char; _Val = std::pair<const char, faker::CharCount>; _KeyOfValue = std::_Select1st<std::pair<const char, faker::CharCount> >; _Compare = std::less<char>; _Alloc = std::allocator<std::pair<const char, faker::CharCount> >]’ at /usr/include/c++/13/bits/stl_tree.h:2532:36,
    inlined from ‘std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::find(const key_type&) [with _Key = char; _Tp = faker::CharCount; _Compare = std::less<char>; _Alloc = std::allocator<std::pair<const char, faker::CharCount> >]’ at /usr/include/c++/13/bits/stl_map.h:1220:25,
    inlined from ‘static std::string faker::String::generateStringWithGuarantee(faker::GuaranteeMap&, std::set<char>&, unsigned int)’ at /home/runner/work/faker-cxx/faker-cxx/src/modules/string/String.cpp:96:37:
/usr/include/c++/13/bits/stl_tree.h:1952:9: error: ‘item’ may be used uninitialized [-Werror=maybe-uninitialized]
 1952 |         if (!_M_impl._M_key_compare(_S_key(__x), __k))
      |         ^~
In file included from /home/runner/work/faker-cxx/faker-cxx/src/modules/string/String.cpp:9:
/home/runner/work/faker-cxx/faker-cxx/include/faker-cxx/Helper.h: In static member function ‘static std::string faker::String::generateStringWithGuarantee(faker::GuaranteeMap&, std::set<char>&, unsigned int)’:
/home/runner/work/faker-cxx/faker-cxx/include/faker-cxx/Helper.h:92:11: note: ‘item’ was declared here
   92 |         T item;
      |           ^~~~
cc1plus: all warnings being treated as errors

Not sure what best approach here, it could ignore Werror (remove from CMake), ignore that specific error (add pragma in String.cpp), or fixing it in the code (I don't know should be fixed in that case.

[cieslarmichal]

I think we can disable werror flag.

[uilianries]

@cieslarmichal Done, without -Werror everything passed.