Add storefront access token service

[chouandy]

Add StorefrontAccessToken support
API docs: https://help.shopify.com/api/reference/access/storefrontaccesstoken

[codecov]

Codecov Report

Merging #19 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #19   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          23     24    +1     
  Lines         890    904   +14     
=====================================
+ Hits          890    904   +14

Impacted Files	Coverage Δ
goshopify.go	100% <100%> (ø)	⬆️
storefrontaccesstoken.go	100% <100%> (ø)

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[rickywiens]

Thanks for the PR! :)