API refactor: Intent logic

[0xGabi]

This issue keeps track of the changes that need to happen related to the Intent entity described on #132.

• [x] New methods to interact with Transaction: sign, transactions
• [x] Update the current path method to use the new data structures and types
• [ ] New method to calculate all the possible Forwarding Paths: paths

[JovianBrowne]

@nivida This issue has no description and no assignees currently. Should it be moved to another column for lower priority issues?

[nivida]

@steffclearteam The detailed description of this task is as mentioned here to find in the issue description of issue #132.