Add PreTokenGeneration & UserMigration Cognito triggers

[v-anton]

What did you implement:

Closes #5513

How did you implement it:

Added PreTokenGeneration & UserMigration triggers to the validTriggerSources since CloudFormation already supports it:

const validTriggerSources = [
  'PreSignUp',
  'PostConfirmation',
  'PreAuthentication',
  'PostAuthentication',
  'PreTokenGeneration',
  'CustomMessage',
  'DefineAuthChallenge',
  'CreateAuthChallenge',
  'VerifyAuthChallengeResponse',
  'UserMigration',
];

How can we verify it:

1. Bootstrap Serverless framework project.
2. Add the following code to serverless.yml:

service:
  name: testservicename
custom:
  stage: ${opt:stage, 'dev'}
  region: ${opt:region, 'eu-central-1'}
provider:
  name: aws
  runtime: nodejs10.x
  stage: ${self:custom.stage}
  region: ${self:custom.region}
functions:
  user-migration:
    handler: user-migration.handler
    events:
      - cognitoUserPool:
          pool: ${self:service.name}-${self:custom.stage}-user-pool
          trigger: UserMigration
          existing: true
  pre-token-generation:
    handler: pre-token-generation.handler
    events:
      - cognitoUserPool:
          pool: ${self:service.name}-${self:custom.stage}-user-pool
          trigger: PreTokenGeneration
          existing: true
resources:
  Resources:
    CognitoUserPool:
      Type: AWS::Cognito::UserPool
      Properties:
        UserPoolName: ${self:service.name}-${self:custom.stage}-user-pool
        UsernameAttributes:
          - email
        AutoVerifiedAttributes:
          - email

    CognitoUserPoolClient:
      Type: AWS::Cognito::UserPoolClient
      Properties:
        ClientName: ${self:service.name}-${self:custom.stage}-user-pool-client
        UserPoolId:
          Ref: CognitoUserPool
        ExplicitAuthFlows: [ADMIN_NO_SRP_AUTH, USER_PASSWORD_AUTH]
        GenerateSecret: false

3. Deploy the app.
4. Check if triggers are referenced properly in AWS Cognito User Pool settings.

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

[pmuens]

Thanks for working on this @v-anton 👍

I just tested it and it worked fine! LGTM

[v-anton]

Thanks @pmuens