Pass in number of partitions to BuildWriter

When implementing a ScanBuilder, we require the implementor to provide the schema of the data and the number of partitions.

However, when someone is implementing WriteBuilder we only pass them the schema, but not the number of partitions. This is an asymetrical developer experience. Passing in the number of partitions on the WriteBuilder would enable data sources to provision their write targets before starting to write. For example, it could be used to provision a Kafka topic with a specific number of partitions.