Identity overrides in local evaluation mode

[khvn26]

1. Extend the Environment model with the identity_overrides: List[IdentityModel] field.
2. On environment update in local evaluation mode, store overrides so they're efficiently accessed by identifier.
3. For getIdentityFlagsFromDocument interface, 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.

Refer to the following existing implementations:

Flagsmith/flagsmith-python-client#72
Flagsmith/flagsmith-java-client#142
Flagsmith/flagsmith-nodejs-client#143