Use StatisticsRegistry from Env (#7166)

Use StatisticsRegistry from Env (#7166)

This commit better integrates the StatisticsRegistry with the environment. It makes the registry an `EnvObj` itself and adds a getter to `EnvObj` to get the registry. It also refactors parts of cvc5 to use this new mechanism to obtain the registry instead of using the (global, static) `smtStatisticsRegistry()` function.