fix: explicitly assign buffer to node.global

[sachscode]

Description of changes

After a discussion with @josefaidt , the Initial change which explicitly initialized global.buffer in polyfills.ts has been removed and the buffer library is now imported through a webpack.config plugin.

Issue #, if available

10304

Description of how you validated changes

amplify mock does not correctly render GraphiQL explorer and Cognito fails OIDC token generation with the error
"Error: Error when generating OIDC token: Buffer is not defined".
The change explicitly defines global ( if missing on platforms ) and assigns buffer to global .
Test:

amplify add api   (graphql + cognito user pools + iam auth)
amplify mock 

and Update graphql schema

type Todo
  @model
  @auth(rules: [{ allow: public }, { allow: private, provider: iam }]) {
  id: ID!
  name: String!
  description: String
}

GraphiQL UI works fine.

Checklist

• [*] PR description included
• [*] yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies

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

[codecov-commenter]

Codecov Report

Merging #10308 (b522199) into master (5483d68) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10308   +/-   ##
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  Coverage   54.35%   54.35%           
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  Files         851      851           
  Lines       46974    46974           
  Branches    10011    10011           
=======================================
  Hits        25532    25532           
  Misses      19427    19427           
  Partials     2015     2015           

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