Multiple event definitions for existing Cognito User Pools

[pmuens]

What did you implement:

Closes #6420

Adds support to define multiple Cognito User Pool events in one function.

How did you implement it:

Fix it and write regression tests.

How can we verify it:

Use the following serverless.yml file or run the Cognito User Pool integration tests (which were updated as well).

service: test-${self:custom.idx}

provider:
  name: aws
  runtime: nodejs10.x
  versionFunctions: false

custom:
  idx: 0
  pool: ExistingUserPool

functions:
  pre:
    handler: handler.hello
    events:
      - cognitoUserPool:
          pool: ${self:custom.pool}
          trigger: PreSignUp
          existing: true
      - cognitoUserPool:
          pool: ${self:custom.pool}
          trigger: DefineAuthChallenge
          existing: true
  post:
    handler: handler.hello
    events:
      - cognitoUserPool:
          pool: ${self:custom.pool}
          trigger: CreateAuthChallenge
          existing: true
      - cognitoUserPool:
          pool: ${self:custom.pool}
          trigger: VerifyAuthChallengeResponse
          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?: NO
Is it a breaking change?: NO

[medikoo]

LGTM!