Check if more than 1 existing bucket is configured

[pmuens]

What did you implement:

Adds a check if more than 1 S3 bucket is being configured.

Related to changes made in #6491

How did you implement it:

Track S3 bucket names while compiling CloudFormation resources.

How can we verify it:

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

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

custom:
  idx: 0
  bucket: serverless-existing-s3-bucket

functions:
  created:
    handler: handler.hello
    events:
      - s3:
          bucket: ${self:custom.bucket}
          existing: true
      - s3:
          bucket: ${self:custom.bucket}-2
          existing: true

should throw, while the following doesn't:

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

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

custom:
  idx: 0
  bucket: serverless-existing-s3-bucket

functions:
  created:
    handler: handler.hello
    events:
      - s3:
          bucket: ${self:custom.bucket}
          existing: true
      - s3:
          bucket: ${self:custom.bucket}
          existing: true

You might also look into the added test case.

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
• 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

[medikoo]

👍