Added Attachment to org package

[samlown]

Adds support for Attachments that are compliant with the EN16931 specification in invoices.

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added thorough tests with at least 90% code coverage.
• I've modified or created example GOBL documents the show my changes in use, if appropriate.
• When adding or modifying a tax regime or addon, I've added links to the source of the changes either structured or in the comments.
• I've run go generate . to ensure that Schemas and Regime data are up to date.
• All linter warnings have been reviewed and fixed.
• I've been obsessive with pointer nil checks to avoid panics.
• The CHANGELOG.md has been updated with an overview of my changes.
• Requested a review from @samlown.

[codecov]

Codecov Report

Attention: Patch coverage is 79.54545% with 9 lines in your changes missing coverage. Please review.

Project coverage is 82.91%. Comparing base (152a0ac) to head (b6adfa3).

Files with missing lines	Patch %	Lines
addons/eu/en16931/org.go	0.00%	7 Missing ⚠️
addons/eu/en16931/en16931.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           cef-vatex     #443      +/-   ##
=============================================
- Coverage      82.92%   82.91%   -0.02%     
=============================================
  Files            242      244       +2     
  Lines          11897    11941      +44     
=============================================
+ Hits            9866     9901      +35     
- Misses          1671     1680       +9     
  Partials         360      360              

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