Add support for existing Cognito User Pools

[pmuens]

This PR adds support for existing Cognito User Pools via a custom resource.

What did you implement:

Closes #4207
Closes #5401
Refs #4241
Refs #5642
Refs #6290

/cc @exoego

How did you implement it:

Checks the existing config property of the cognitoUserPool event and swaps out the Cognito User Pool CloudFormation resource with a custom resource the Serverless Framework provides. The custom resource basically does raw SDK calls to update the existing Cognito User Pool. This way everything is encapsulated via CloudFormation and the stack won't get out of sync if we implement it via raw SDK calls.

How can we verify it:

service: test

provider:
  name: aws
  runtime: nodejs10.x

functions:
  test:
    handler: handler.hello
    events:
      - cognitoUserPool:
          pool: ExistingUserPool
          trigger: CustomMessage
          existing: true

Todos:

Note: Run npm run test-ci to run all validation checks on proposed changes

• Write tests and confirm existing functionality is not broken.
  Validate via npm test
• Write documentation
• Ensure there are no lint errors.
  Validate via npm run lint-updated
  Note: Some reported issues can be automatically fixed by running npm run lint:fix
• Ensure introduced changes match Prettier formatting.
  Validate via npm run prettier-check-updated
  Note: All reported issues can be automatically fixed by running npm run prettify-updated
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[pmuens]

Thanks for the reviews @medikoo and @exoego 👍

I just addressed your issues and updated the PR accordingly. Other than that I also added support for the usage of multiple custom resources in one service.

[medikoo]

LGTM, still on stylistic note, it'll be good to reduce lodash usage.

I see in many places _.merge where Object.assign works, or _.forEach(array), where we can directly do array.forEach or for (const item of array)

I think it'll be good to favor native functions over external utils (where they do not bring added value)

[pmuens]

Thanks for the review @medikoo 👍

I agree that we should definitely try to adhere to vanilla JS as much as possible given that we dropped support for all the old Node.js versions.