List<AWS::EC2::Subnet::Id> should sort subnet names by VPC or group by VPC or allow filter by VPC.

### Name of the resource

AWS::EC2::Subnet

### Resource name

List

### Description

When List resource is used in cloudformation template, it is really hard to select right subnets where resource such as MSK, DocumentDb or AWS RDS needs to be provisioned as they are neither following logical order nor does it group based on VPC.

Hence feature request is to allow:
a. filtering subnets based on previously selected VPC
OR
b. group subnets by VPC (HTML has optGroup tag - refer attached mockup)
e.g.
VPC1
- vpc1_subnet1
- vpc1_subnet2
VPC2
- vpc2_subnet1
- vpc2_subnet2
OR
c. sort subnets by VPC so, all related subnets in vpc are together

### USECASE:

For instance, imagine if you have 10 VPCs with 3 AZ, 4 subnets per AZ that comes to 120 subnets (10 X 3 X 4)in a single list and selecting right subnets becomes a task. This feature will enhance filtering subnets based on previously selected VPC, group subnets or sort subnets by VPC so, all related subnets in vpc are together.

### Other Details

Kindly Refer subnet-list.png where I need to list scroll up and down to locate right subnets for my selected VPC.

Start with the AWS::EC2::Subnet resource and the List CloudFormation parameter described in the issue. Compare the requested alternatives—filtering by a selected VPC, grouping with optGroup, or sorting by VPC—and use the supplied subnet-list and mockup images to understand the intended selection experience. Done means subnet selection makes related VPC subnets easier to identify in large lists.

Written by the indexing model from the issue text.