Missing Substrate Methods

[thewhaleking]

Numerous methods are missing in async-substrate-interface that exist in py-substrate-interface:

• subscribe_storage
• get_metadata_constants
• get_metadata_event
• create_unsigned_extrinsic
• get_block_runtime_version
• create_multisig_extrinsic
• get_metadata_events
• get_storage_by_key
• get_metadata
• generate_multisig_account
• retrieve_pending_extrinsics
• get_metadata_call_functions