AWS::DynamoDB::Table GlobalSecondaryIndex Support for Multiple GSIs

## 2. Scope of request
Cloudformation cannot perform more than one DynamoDB GSI creation or deletion in a single update. Instead, users must add/delete a single GSI per update, and then manually create a new update for each GSI which needs to be added. For teams which have Cloudformation run as part of a deployment pipeline, this is even more bothersome as it necessitates waiting for multiple end-to-end runs of the entire deployment pipeline for each GSI that needs to be created/deleted.

Furthermore, once a Cloudformation stack includes a DynamoDB table with more than one GSI, it is no longer possible for someone to re-use this stack without manually editing it and going though the one-at-a-time GSI update process. This issue seems to be in direct contrast with the advertised goals of CloudFormation.

https://aws.amazon.com/cloudformation/
> AWS CloudFormation provisions your resources in a safe, repeatable manner, allowing you to build and rebuild your infrastructure and applications, without having to perform manual actions or write custom scripts.

## 3. Expected behavior

1. Update operations to existing DynamoDb tables succeed even when adding/deleting multiple GSIs
2. Creating a new DynamoDb table using an Cloudformation stack that includes multiple GSIs succeeds

## 4. Suggest specific test cases

1. Creating a new DynamoDb table with two GSIs is able to complete successfully
2. Adding two GSIs in a single update to an existing table is able to complete successfully
3. Deleting a DynamoDb table with two GSIs is able to complete successfully
4. Removing two GSIs in a single update to an existing table is able to complete successfully

## 5. Helpful Links to speed up research and evaluation

https://stackoverflow.com/questions/28402003/creating-multiple-gsis-by-updatetable-dynamodb
https://stackoverflow.com/questions/47920665/how-to-delete-more-than-one-global-secondary-index-from-the-cloudformation-at-on
[Managing Global Secondary Indexes documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GSI.OnlineOps.html)
## 6. Category

DB: DynamoDb