missing ability to read/write GCS object metadata

[BorisDaich]

please add an option to read/write GCS object metadata

• write metadata along with creation of a new GCS object.
• full/partial metadata update for existing GCS object.
• Read metadata along with the GCS object read.
• Read metadata without reading the GCS object itself.

high level proposed addition:

depending on the boolean flag in the node that can be overridden by the flags in the message "readMetadata" for the GCS Read node and "writeMetadata" GCS Write node

absence of the flag in the message considered as "false".

If the flag is set to "true" than GCS Read will return the GCS Object's metadata in msg.metadata
GCS Write will write the msg.metadata along with the object. if metadata provided violates GCS metadata rules - the node should through a relevant exception.
on write if both msg.metadata and msg.contentType contain contentType than the msg.contentType should prevail.

Open Questions that the proposal does not address:

Update of the metadata for existing GCS object
reading metadata without reading the GCS object itself.