What this PR does / why we need it:
Makes Projects optional by introducing a default project default:

• Feast Core automatically creates a non archivable default project.
• default project would be used where users do not explicit specify a project in CoreService's listFeatureSets(), getFeatureSet(), applyFeatureSet().
• Feast Serving resolves Feature References without projects specified to Features in the default projects. The project field is now optional.

Update Feature References to support duplicate Features names within project but unique within Feature Set:

• Introduces optional feature_set field to FeatureReference proto.
• Allows users to specify feature set name by extending string Feature References to the format: [project/][feature_set:]feature_name with Feast Serving
• Feast Serving attempts to resolve the feature references with missing project, and /or feature_set_name. If multiple features match, returns an error to mark the ambiguity.
• Versions in Feature References are ignored by serving if specified.

When specifying Feature Set name in string feature references to get feature data (ie feature_set:feature)

• Online Serving returns the feature data keyed by the submitted string feature reference.
• Batch Serving returns the feature data in columns named in the format feature_set__feature

Serving attempts to maintain backwards compatibility with existing feature references in format: [project/]feature

Update Feast Go, Java, Python SDKs to transition to string Feature References without Projects:

• Remove support for specifying projects in string feature references in SDKs. Specifying project in string Feature References supplied is now an error.
• Add project field to get batch/online features methods to allow users to explicitly specify project.
• String Feature References returned from get online features methods no longer contain project.

Which issue(s) this PR fixes:

Fixes #631

Does this PR introduce a user-facing change?:

Projects are now optional in Feast APIs (except ingestion).
- Feast creates a "default" project automatically.
- Feast uses the the "default" project where a project is not explicitly specified. 

Feast now namespaces Feature names within its Feature Set.
- Added optional "feature_set_name" field to FeatureReference proto.
- Feast supports FeatureSet specifying Feature Sets Name in string feature references: "driver_fs:driver_feature"
- Feast now allows features to have a unique name within a FeatureSet, but duplicated across a Project

Action Required: Dropping Support for specifying projects in string Feature References in Feast SDKs:
- Projects now cannot be specified in string Feature References passed to get online/batch features methods.
- Use the project parameter in those methods to specify projects instead. 
- String Feature References returned in get online features will not contain projects.