Provide application interface information

[shuse2]

Description

Expose an endpoint to return metadata describes the application interface including

• schema for block, transaction, params for commands, event and block assets
• endpoints
• registered events

Motivation

Information for external system, such as lisk-service or lisk-desktop, to know what commands, endpoints, assets or events are available in the connected application should be exposed through the RPC

Tasks

• Update all modules to expose metadata #7063
• Namespace RPC endpoint and add new endpoints #6991
• Add system_getMetadata endpoint #7064
• Add node:module-metadata command #7065