fix: transaction sign and OP_RETURN + import

[Alex-Werner]

Issue being fixed or feature implemented

What was done?

• allow .sign() method to auto-sign transaction based on inputs.
• allow importTransaction() to import JSON based data
• allow .sign() to handle cases with OP_RETURN (account.sign(transaction) throws error 'Require one or multiple privateKeys to sign' #160)

How Has This Been Tested?

Added provided tests

Breaking Changes

N/A

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

[antouhou]

Seems great! I can't wrap my head around why the commits from the #185 ended up in this PR though 🤔 I guess 0.16 was started before we've merged the #185 into the master

[antouhou]

I've updated the 0.16 branch, so now those commits from #185 are gone from this PR 👍

Fixed by updating 0.16 branch as it was master commit

[antouhou]

uTACK 👍