Add data as output (via OP_RETURN)

[RCasatta]

Description

This PR add a method fn add_data(&mut self, data: &[u8]) to the TxBuilder as a shortcut to build an op_return output.

Notes to the reviewers

To allow sending outputs with 0 value, the dust check has been modified, excluding from the check scripts that are provably unspendable.

As briefly discussed in the call, while using op_return may be controversial, it exists and it's always better than the alternative for embedding data. For this reasons wallet like bitcoin core and electrum allow to create transactions with it

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature
• I've updated CHANGELOG.md

[RCasatta]

I had to rebase because of a conflict in CHANGELOG

[notmandatory]

ACK aa075f0