feat(@aws-amplify/api): pass additionalHeaders to graphql function

[jensbodal]

Additional headers are merged after merging in headers set at config time.
Additional headers will overwrite any existing values and merge the rest.

Issue #, if available: #4981

Description of changes: Existing private _graphql function allowed passing a param called additionalHeaders but was never used. The public graphql function now accepts an optional additionalHeaders param and is passed to private _graphql method. _graphql method was modified to merge in additionalHeaders after merging in previously set headers. This is how I would expect the parameter to function.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@8f6acca). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #5001   +/-   ##
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  Coverage          ?   76.39%           
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  Files             ?      175           
  Lines             ?     9670           
  Branches          ?     1982           
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  Hits              ?     7387           
  Misses            ?     2137           
  Partials          ?      146           

Impacted Files	Coverage Δ
...aws-amplify-react/src/Auth/Provider/withGoogle.tsx	84.61% <0.00%> (ø)
packages/analytics/src/trackers/PageViewTracker.ts	88.31% <0.00%> (ø)
...kages/interactions/src/Providers/AWSLexProvider.ts	76.27% <0.00%> (ø)
packages/xr/src/Errors.ts	100.00% <0.00%> (ø)
...pubsub/src/Providers/AWSAppSyncRealTimeProvider.ts	18.55% <0.00%> (ø)
packages/core/src/Util/Retry.ts	94.11% <0.00%> (ø)
packages/storage/src/Storage.ts	91.01% <0.00%> (ø)
packages/core/src/Credentials.ts	33.17% <0.00%> (ø)
packages/core/src/ClientDevice/index.ts	71.42% <0.00%> (ø)
packages/core/src/ClientDevice/browser.ts	9.09% <0.00%> (ø)
... and 165 more

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[manueliglesias]

Thank you for the contribution, looks great! 👍 🎉

[bensie]

@manueliglesias @jordanranz It looks like the merge of #5215 accidentally stomped on this change so additionalHeaders are no longer working. Can we get the change added back?

[bensie]

After more digging I see that API.graphql(...) is deprecated in favor of GraphQLAPI.graphql(...) and the latter supports additionalHeaders (though not for subscriptions - see #5576). This still appears to be an accidental merge error, but switching to the new GraphQLAPI is pretty straight forward so it's not critical to get this fixed. That said, the inline documentation still indicates that additionalHeaders are supported on API.graphql(...), so at the very least a docs cleanup is probably warranted.

[github-actions]

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