Allow passing custom PRNG to generate masks

[stackinspector]

It is possible to include a place for RNG state in the appropriate place and accept any rand_core::RngCore + rand_core::CryptoRng when creating protocol state. But the amount of work to make this change seems a bit much. In addition, if I use ws without handshake and both server and client agree, masks seem to become useless (since no http is involved). Given that handshake is already an optional feature, can we consider making mask an optional feature as well? (This request may be exceeded.)