Refactor: refactored tangle plugin

packages/model/approvers/approvers.go

packages/model/transactionmetadata/const.go

packages/model/transactionmetadata/errors.go

packages/model/transactionmetadata/transactionmetadata.go

plugins/tangle/api.go

plugins/tangle/approvers.go

plugins/tangle/plugin.go

plugins/tangle/solidifier.go

plugins/tangle/solidifier_test.go

plugins/tangle/transaction.go

plugins/tangle/transaction_metadata.go