feat(sdk)!: Do not set metadata defaults in Stream constructor

[teogeb]

This is a breaking change as the value returned by getMetadata() is different.

The Stream constructor no longer sets default values for any metadata fields. Before this PR the constructor initialized values for partitions and fields values. As metadata is now untyped (#2845), it makes sense to drop these defaults.

Also the internal Streamr#parseMetadata() no longer sets default value for the partitions field .

Implications

Stream#getMetadata() now returns metadata exactly as it stored in the stream registry.

• But we can easily query the partition count with the Stream#getPartitionCount() method. It defaults to the DEFAULT_PARTITION_COUNT if there is no value available in metadata.
• And very typically the partition count has been explicitly stored to stream registry metadata, so it is available also when queried via Stream#getMetadata().partitions.