feat(tax): adds getItemTaxLines

[srindom]

What

• Selects the correct tax line for an item given a calculation context.

For later PR

• Consider optimizations. Some thoughts:
  • Even with global sales the number of rates in the DB is not likely to grow beyond ~1000.
  • Can large orders with hundreds of items optimize somehow?
  • Does it make sense to write a custom SQL query to do this?
• Support combined rate.

Test cases covered
The selection of tax rates take the following priority:

1. specific product rules - province
2. specific product type rules - province
3. default province rules
4. specific product rules - country
5. specific product type rules - country
6. default country rules

There are test cases for each of them under the following data seed structure:

US

• Default Rate: 2%
• Sub-Regions
  • CA
    • Default Rate: 5%
    • Overrides
      • Reduced rate (for 3 product ids): 3%
      • Reduced rate (for product type): 1%
  • NY
    • Default rate: 6%
  • FL
    • Default rate: 4%
• Overrides
  • None

Denmark

• Default rate: 25%
• Sub-Regions
  • None
• Overrides
  • None

Germany

• Default Rate: 19%
• Sub-Regions
  • None
• Overrides:
  • Reduced Rate (for product type) - 7%

Canada

• Default rate: 5%
• Sub-Regions
  • QC
    • Default rate: 2%
    • Overrides:
      • Reduced rate (for same product type as country reduced rate): 1%
  • BC
    • Default rate: 2%
• Overrides
  • Reduced rate (for product id) - 3%
  • Reduced rate (for product type) - 3.5%

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[riqwan]

Readability takes a bit of a hit here with the multi-nests. LGTM otherwise with some comments.

[adrien2p]

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