Identity overrides in local evaluation mode

[khvn26]

After Flagsmith/flagsmith-rust-flag-engine#11 is done and released:

1. On environment update in local evaluation mode, store overrides so they're efficiently accessed by identifier.
2. For ‎Flagsmith::build_identity_model interface (now more appropriate to call it get_identity_model), use the storage above to retrieve the identity overrides. Fall back to a new IdentityModel instance if not found. If found, update traits with user-provided traits.