feat: use single random engine per thread and provide ability to set seed

[Flogex]

This PR makes it possible to set the seed of the random engine by calling the static method faker::common::SetSeed. Similar to std::mt19937::seed, the method can either take a scalar value or a seed_seq.

Previously, std::mt19937 random engines where either global static or instantiated on every method call. I changed the code to only use a single random engine per thread, a reference to which can be obtained by calling faker::common::GetGenerator. The random engine is thread local to avoid race conditions in case multiple threads try to access it in the future.

SetSeed sets the seed of the thead-local instance corresponding to this thread. If SetSeed was not called before, the seed will be obtained from std::random_device. In the current code, std::random_device is used as well to seed the random engine, probably under the assumption that its entropy is always greater than zero.

To be able to replace all instantiations of std::mt19937, I added a new constructor to RandomGenerator that takes copies an existing random engine. Furthermore, the 64-bit version is now used everywhere because faker::date::anytime uses it already and I opted to not change this. I also didn't want to duplicate all members and methods to provide a 32 and a 64 bit version of each. If you have performance concerns when using mt19937_64, I'm fine with changing it to using mt19937 everywhere including in date::anytime.

Closes #1012

[cieslarmichal]

please fix build