Flatten configuration structure for online store

[woop]

Signed-off-by: Willem Pienaar git@willem.co

What this PR does / why we need it:

The current configuration structure requires a nested online_store configuration like this

project: foo
registry: "registry.db"
provider: local
online_store:
    local:
        path: online_store.db

This refactor moves to

project: foo
registry: "registry.db"
provider: local
online_store:
    type: sqlite
    path: online_store.db

but allows for a default type to be set, based on the provider.

project: foo
registry: "registry.db"
provider: local
online_store:
    path: online_store.db

Much of the work in this PR is just to ensure that exceptions are clear to the user.

Does this PR introduce a user-facing change?:

Flattened configuration structure for online store

[jklegar]

I think we'll need defaults for provider and project in order for RepoConfig() with no arguments to work

[jklegar]

fyi there's an empty file sdk/python/telemetry_tests/data/online.db included in here

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jklegar, woop

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[jklegar]

/lgtm