Extend payment gateway GraphQL type

[salwator]

Fixes #4772

Roadmap:

• Extend GraphQL schema
• Add new plugin method
• Serve config from gateways:
  • Dummy (sample store_customer)
  • Stripe (api_key, store_customer)
  • Braintree (client_token, store_customer)
• Align tests and types

Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. GraphQL schema and type definitions are up to date.
8. Changes are mentioned in the changelog.

[django-queries]

Here is the report for 346bc46 (mirumee/saleor @ extend-gateway-graphql-type)
Base comparison is a54cbf6.

**Found 2 differences!** (click me)

# api.benchmark checkout
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
+ add billing address to checkout            	         38	         34	             20
  add shipping to checkout                   	          7	          7	              0
  checkout payment charge                    	         14	         14	              0
  complete checkout                          	          6	          6	              0
+ create checkout                            	         52	         48	             24

# 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                            	         15	         15	              3
  retrieve product attributes                	         13	         13	              2

# api.benchmark variant
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  retrieve variant list                      	         18	         18	              8

# api product sorting attributes
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  sort product not having attribute data     	         21	         21	              0

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a54cbf6). Click here to learn what that means.
The diff coverage is 72.72%.

@@            Coverage Diff            @@
##             master    #4774   +/-   ##
=========================================
  Coverage          ?   91.23%           
=========================================
  Files             ?      343           
  Lines             ?    20551           
  Branches          ?     1954           
=========================================
  Hits              ?    18749           
  Misses            ?     1264           
  Partials          ?      538

Impacted Files	Coverage Δ
saleor/payment/gateways/braintree/plugin.py	0% <0%> (ø)
saleor/payment/gateways/stripe/plugin.py	0% <0%> (ø)
saleor/extensions/base_plugin.py	90.16% <100%> (ø)
saleor/payment/gateways/stripe/__init__.py	98.87% <100%> (ø)
saleor/graphql/checkout/types.py	94.73% <100%> (ø)
saleor/extensions/manager.py	88.2% <100%> (ø)
saleor/payment/gateways/braintree/__init__.py	93.75% <100%> (ø)
saleor/payment/gateways/dummy/plugin.py	92.3% <100%> (ø)
saleor/payment/gateway.py	95.52% <100%> (ø)

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