Add bill line charge rate and quantity

[samlown]

• Support in bill lines for charges with a Rate and optional Quantity for special types of tariffs to apply to the price.

Pre-Review Checklist

• 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 to 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 the 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

All modified and coverable lines are covered by tests ✅

Project coverage is 87.38%. Comparing base (5e7b060) to head (5f65391).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           fix-rounding-lines     #487      +/-   ##
======================================================
+ Coverage               87.30%   87.38%   +0.08%     
======================================================
  Files                     264      264              
  Lines                   13538    13628      +90     
======================================================
+ Hits                    11819    11909      +90     
  Misses                   1373     1373              
  Partials                  346      346              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.