Changes to Get topic message by consensusTimestamp

changed the pattern of the path request parameter timestamp from ^\d{1,10}(.\d{1,9})?$ to ^\d{1,10}(\.\d{1,9})?$

the response property _status/messages/items/data became nullable for the status 400

the response property _status/messages/items/data became nullable for the status 404

the response property _status/messages/items/detail became nullable for the status 400

the response property _status/messages/items/detail became nullable for the status 404

the response property chunk_info became nullable for the status 200

the response property chunk_info/initial_transaction_id/account_id became nullable for the status 200

the response property chunk_info/initial_transaction_id/nonce became nullable for the status 200

the response property chunk_info/initial_transaction_id/scheduled became nullable for the status 200

the response property payer_account_id became nullable for the status 200

the response property topic_id became nullable for the status 200

changed the pattern of the path request parameter timestamp from ^\d{1,10}(\.\d{1,9})?$ to ^\d{1,10}(.\d{1,9})?$