Add tax domain

[arjun289]

Why?

At present tax needs to be handled for product and shipping. The PR adds tax domain module which handles all the business logic and edge cases related to tax and is used in respective domain modules for package item and package to set taxes for product and shipping.

This change addresses the need by:

• Adds tax_class field to products so now all the products have a belongs to relationship with tax_class.

  product bleongs_to tax_class

  This relationship allows tax calculation for a product by findind respective values in a tax rate for that particular tax_class.

• Unit price field has been added to package_item this was done so as to keep the actual price after tax calculation which is exclusive of the tax.
• Adds tax domain to handle all the business logic. At present tax domain handles tax calculation for product and shipping.

[delivers #163283496]

Checklist

• [x] I have read CONTRIBUTING.md.
• [x] My code follows the style guidelines of this project.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] My code does not generate any (new) credo and compile-time warnings.
• [x] I have updated the documentation wherever necessary.
• [x] I have added tests to cover my changes.

[codecov[bot]]

Codecov Report

Merging #465 into develop will increase coverage by 0.62%. The diff coverage is 95.94%.

@@             Coverage Diff             @@
##           develop     #465      +/-   ##
===========================================
+ Coverage     78.4%   79.02%   +0.62%     
===========================================
  Files          174      175       +1     
  Lines         2139     2198      +59     
===========================================
+ Hits          1677     1737      +60     
+ Misses         462      461       -1

Impacted Files	Coverage Δ
apps/snitch_core/test/support/factory/factory.ex	88.13% <ø> (+5.59%)	:arrow_up:
...s/snitch_core/lib/core/data/schema/package_item.ex	88.88% <ø> (ø)	:arrow_up:
...b/core/domain/calculator/default_tax_calculator.ex	100% <100%> (ø)	:arrow_up:
apps/snitch_core/lib/core/domain/package.ex	81.25% <100%> (+1.25%)	:arrow_up:
apps/snitch_core/lib/core/domain/shipment.ex	98.36% <100%> (+0.02%)	:arrow_up:
apps/snitch_core/lib/core/data/model/product.ex	97.24% <100%> (+0.16%)	:arrow_up:
apps/snitch_core/lib/core/domain/package_item.ex	100% <100%> (ø)	:arrow_up:
...s/snitch_core/lib/core/domain/order/transitions.ex	42.39% <100%> (ø)	:arrow_up:
apps/snitch_core/lib/core/data/schema/product.ex	87.5% <75%> (-2.16%)	:arrow_down:
apps/snitch_core/lib/core/domain/tax.ex	96.22% <96.22%> (ø)
... and 1 more

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