kvs-watch: support RAW option?

[chu11]

Per comments in #6523 (comment), the treeobj_create_val() can be quite costly for large amounts of data, especially due to the base64 encoding that goes on internally.

In some cases, return the raw value instead of treeobject might be warranted for performance and convenient. FLUX_KVS_STREAM might be such a case, since it is looking up individual blob refs rather than the true "combined" value from the KVS.