Enable extension manager for taxes

[korycins]

Convert all taxes logic into a plugin architecture

• Convert vatlayer into Plugin
• Convert avatax into Plugin
• Enable extensions manager

It closes #3992

[korycins]

@salwator @maarcingebala @NyanKiyoshi All logic from saleor.core.extensions.plugins.avatax, saleor.core.extensions.plugins.vatlayer directories has been moved from the taxes.interface module.

[django-queries]

Here is the report for e875d72 (korycins/saleor @ enable_extension_manager_for_taxes)
Base comparison is 0a28e71.

No differences were found. (click me)

# api.benchmark checkout
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  add billing address to checkout     	         34	         34	             20
  add shipping to checkout            	          7	          7	              0
  checkout payment charge             	         14	         14	              0
  complete checkout                   	          6	          6	              0
  create checkout                     	         41	         41	             20

# api.benchmark homepage
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve main menu                  	          5	          5	              0
  retrieve product list               	          4	          4	              0
  retrieve secondary menu             	          5	          5	              0
  retrieve shop                       	          2	          2	              0

# api.benchmark product
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  product details                     	         13	         13	              3

# api.benchmark variant
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve variant list               	          9	          9	              2

[codecov]

Codecov Report

Merging #4497 into master will increase coverage by 0.19%.
The diff coverage is 66.47%.

@@            Coverage Diff             @@
##           master    #4497      +/-   ##
==========================================
+ Coverage   90.62%   90.82%   +0.19%     
==========================================
  Files         297      297              
  Lines       17434    17469      +35     
  Branches     1746     1745       -1     
==========================================
+ Hits        15800    15866      +66     
+ Misses       1122     1092      -30     
+ Partials      512      511       -1

Impacted Files	Coverage Δ
saleor/core/views.py	92.85% <ø> (ø)	⬆️
saleor/product/utils/__init__.py	95.29% <ø> (ø)	⬆️
saleor/graphql/payment/mutations.py	97.7% <ø> (-0.03%)	⬇️
saleor/core/templatetags/taxed_prices.py	88.88% <0%> (+7.93%)	⬆️
...aleor/core/extensions/plugins/vatlayer/__init__.py	95.52% <100%> (ø)
saleor/dashboard/order/forms.py	87.43% <100%> (+0.06%)	⬆️
saleor/product/utils/availability.py	97.33% <100%> (+0.15%)	⬆️
saleor/checkout/forms.py	94.47% <100%> (+0.09%)	⬆️
saleor/dashboard/product/views.py	93.36% <100%> (-0.02%)	⬇️
saleor/checkout/views/__init__.py	86.13% <100%> (+0.31%)	⬆️
... and 32 more

---

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[patrys]

Looks great but seems there are no documentation changes?

[korycins]

@maarcingebala @patrys Documentation has been added.

[patrys]

I think we should start with "how to use it" before we dive into the details of implementing a custom plugin and eventually a custom manager (not sure if we should event document that part).

[patrys]

I think https://docs.djangoproject.com/en/2.2/topics/http/middleware/ is a good reference for what type of information we would need to provide (but I would still document installing plugins before writing custom ones).

[korycins]

@patrys I've changed the structure as you said. Can you take a look one more time?