[Elephant] Metadata transaction types implementation

[evias]

Metadata features are being discussed in following NIP discussion thread: https://github.com/nemtech/NIP/issues/8

Following task assignment describes the Metadata key/value association features in the catapult-server project: https://github.com/nemtech/catapult-server/issues/17

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Implementation will need the following modules to be added/modified:

• Add interface for MetadataTarget
• Make Account, Mosaic and Namespace implement MetadataTarget
• Add new transaction type MetadataTransaction with following transaction data:

Field	Type	Description
Target Type	enum	Type of metadata target
Target Id	union{signer, targetType, targetId, metadataKey}	Identifier of the metadata target
Metadata key	uint64_t	Metadata identifier
Metadata value	byte*	Metadata payload (transaction.message-alike)
Metadata value hash	hash_256	Hash of the payload to allow pruning. If payload size is less than 32 bytes, it will be inlined in this field (prevents pruning)

• Additionally, MetadataTransaction will be possible only within AggregateBonded transaction because metadata assignments needs opt-in logic.

• Default maximum payload content size 1024 bytes (transaction.message alike)

• Discuss with catapult-rest team to provide with MetadataHttp implementation with :

  • MetadataHttp.getMetadata(targetType, targetId, metadataKey)
  • MetadataHttp.getMetadataFromIssuer(issuer, targetType, targetId, metadataKey)