Add feature flags for specific metadata providers

[zfarrell]

After #8 is implemented, we should support feature flags to allow users to specify an exclusive metadata provider and avoid bringing in unnecessary dependencies. For example:

Only support duckdb metadata provider

datafusion-ducklake = { version = "1", features = ["duckdb"] }

Only support postgres metadata provider

datafusion-ducklake = { version = "1", features = ["postgresql"] }