Add `allow_client_traits_in_evaluation` to environment schema

[khvn26]

Add a new bool field allow_client_traits_in_evaluation (default True) to the environment schema in flagsmith-common. This field controls whether client-sent traits are used for segment evaluation, separately from allow_client_traits which controls trait persistence.

The default of True preserves backwards compatibility — existing environments with allow_client_traits=False will continue using client-sent traits for evaluation until explicitly opted out.

Release a new flagsmith-common version with this change.