Feature request: Add metadata on STORE message

[BjrInt]

Currently the only way to annotate a STORE message is to create an additional POST or AGGREGATE message to provide some context about the stored content.

Some example use-cases for metadata on STORE message :

• Provide a way to name Immutable Volumes (for easy reusage)
• Create a "basic filesystem" on top of IPFS to organize stored files

Metadata are already in use in PROGRAM message, this logic could be reused.