Migrate external transaction helpers for legacy bridge type

[stevenkhong]

Summary

Move bridge transaction helper clients from legacy js-sdk to the @demex-sdk/polynetwork package.

Goals

• Retain on @demex-sdk the functionality to support legacy bridge.

Non-goals

• Due to limited support of the bridge in favour of new bridge type, we do not want to devote extra effort to rewrite these clients.

Tasks

Clients to migrate:

• ETHClient
• N3Client
• NEOClient
• ZILClient