Add affiliate code

[JacobPlaster]

Description:

This pull request add the ability to specify aff_code (affiliate code) when submitting a new order via both rest and websocket. Also the meta field is now extracted from the response and exposed from order object.

Breaking changes:

None

New features:

• [x] Ability to set aff_code
• [x] Meta field exposed in order

Fixes:

None

PR status:

• [x] Version bumped
• [x] Change-log updated
• [x] Docs updated