[Discuss] Feast Enhancements

My team has been evaluating Feast for adoption and have identified a few improvements we'd like to contribute for our use cases. @woop suggested starting a thread for discussion to make sure they are a good fit.

### Support for SPI extensions
We'd like to add extension points for our existing infrastructure for things like:
* FeatureTable change detection (notifying existing data pipeline and catalog services when a feature table is created or modified)
* Implementing online storage operations for our store (HBase)

### Multi-tenancy support
Our infrastructure is entirely multi-tenant. We authorize API calls and store data on a per-tenant basis.
On the surface, we'd need something like the following in Feast:
* Ability to define an optional `TenantSpec tenant` attribute in `FeatureTableSpec` that defines the name + type of a tenant field for feature table keys (e.g., name = 'TenantId'; type = 'string')
* Adding an optional `tenant` attribute to ingestion and serving APIs (+SDK methods)
* Transparently adding the tenant to storage operations when the tenant attribute is set (e.g., including 'TenantId=foo' in row keys)

### Direct online write support
We have use cases that would like to write to feature store directly. Adding an `ApplyOnlineFeatures` API (+SDK support) would satisfy these use cases. The API would semantically resemble the `GetOnlineFeaturesV2` API (write feature table values for specific entity rows).

### Delete online feature support
We maintain GDPR compliance by propagating delete record signals from upstream data systems. Essentially we'd need a `DeleteOnlineFeatures` API (+SDK support) that resembles `GetOnlineFeaturesV2` and `ApplyOnlineFeatures` (delete feature table values for specific entity rows).

If these seem like reasonable enhancements to Feast we can file individual issues and PRs to contribute these incrementally.