Entity Mapping for Feature View

## Expected Behavior
with_feature_view function inside feature_view.py stores the entity mapping provided the user. The feature_view proto format should also have join_key_map field to add this mapping.

## Current Behavior
However since feature_view proto does not have join_key_map field, while converthing this feature_view ot feature_view_proto, this mapping information is lost. And since proto format is stored in registry, while reading the feature view , user also is unable to access that mapping.

## Steps to reproduce

fv = store.get_feature_view(name="fv1")
print(fv.projection)

returns :
FeatureViewProjection(name='fv1', name_alias=None, desired_features=[], features=[display_name-String], join_key_map={})
as visible, joinkey map is empty, where we lost the mapping

### Specifications

- Version: current latest version
- Platform: independent of platform
- Subsystem: independent of any subsystem

## Possible Solution
Introduce join_key_map in proto file for feature_view also, and write rule for adding that map in to_proto and from_proto function in feature_view.py