Improvements on shipping validation

[jxltom]

• Reuse is_valid_shipping_method in API (clean_checkout) and storefront 1.0 (clean_shipping_method)
• Split clean_shipping_method to clean_shipping_method and clean_shipping_address. They are used in CheckoutLinesAdd/CheckoutLineUpdate/CheckoutLineDelete/CheckoutShippingAddressUpdate/CheckoutShippingMethodUpdate. I feels both address and method should be validated after cart line/shipping method/shipping address are updated.
• Fix a bug where address and method should be validated after cart line is updated in CheckoutLinesAdd/CheckoutLineUpdate
• Fix a bug where checkout is not returned if method/address is invalid in CheckoutLinesAdd/CheckoutLineUpdate
• Fix a bug where shipping method should be deleted only when it's invalid in clean_shipping_method
• Fix a bug where is_valid_shipping_method will raise error if cart doesn't have a shipping address
• Fix bug where Country should be used instead of string for address's country field in tests.

Pull Request 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. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@e81494c). Click here to learn what that means.
The diff coverage is 92.1%.

@@            Coverage Diff            @@
##             master    #3812   +/-   ##
=========================================
  Coverage          ?   91.61%           
=========================================
  Files             ?      277           
  Lines             ?    15153           
  Branches          ?     1489           
=========================================
  Hits              ?    13883           
  Misses            ?      870           
  Partials          ?      400

Impacted Files	Coverage Δ
saleor/checkout/views/__init__.py	84.37% <ø> (ø)
saleor/checkout/utils.py	82.17% <87.5%> (ø)
saleor/graphql/checkout/mutations.py	95.7% <93.33%> (ø)

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[jxltom]

@auvipy Is this an auto reply? 😄

[auvipy]

please rebase.

[jxltom]

@maarcingebala This PR is updated with master branch, please review. Thanks.

[maarcingebala]

Closing in favor of #4367 where we're working on improved behavior of checkout API.