AWS::Route53Resolver::ResolverEndpoint -> Add GetAtt support for rni-ids

## AWS::Route53Resolver::ResolverEndpoint -> Add GetAtt support for rni-ids

Quick Sample Summary:
1. AWS::Route53Resolver::ResolverEndpoint -> Add GetAtt support for rni-ids
2. AWS::Route53Resolver::ResolverEndpoint does not support Fn::GetAtt for the list of rni-ids that support the outbound or inbound endpoint.
3. Expected behavior -> The GetAtt intrinsic function should return a list of the network interface identifiers for the endpoint. These identifiers can be used in CloudWatch dashboards to graph the OutboundQueryVolume and OutboundAggregateQueryVolume for each interface.

5. https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-route53resolver-resolverendpoint.html
https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/monitoring-resolver-with-cloudwatch.html

6. Category tag (optional) -> Networking & Content Delivery
7. Any additional context (optional)

## 2. Scope of request
Please add a new attribute for AWS::Route53Resolver::ResolverEndpoint resources to support returning the list of resolver network interface IDs

#### AWS::Route53Resolver::ResolverEndpoint - RniIds - to return a list of resolver network interface identifiers using GetAtt so they can be passed to nested stacks, or to create and store as an SSM param.

## 3. Expected behavior

Samples:
#### The attribute should always return the current list of network interface identifiers provisioned to support the Resolver Endpoint.

We want to build and share CloudWatch Dashboards to graph metrics for OuboundAggregateQueryVolume for each network interface bound to a Resolver Endpoint. It is not possible to build these dashboards with CloudFormation today because there is no way to get the list of the Resolver Network Interfaces without building a custom resource.

```
{
"metrics": [
[ { "expression": "ANOMALY_DETECTION_BAND(m1, 2)", "label": "Expression1", "id": "e1", "region": "us-east-1" } ],
[ "AWS/Route53Resolver", "OutboundQueryAggregateVolume", "EndpointId", "rslvr-out-xxxxxxxxxxxxxx", { "id": "m1", "label": "Endpoint-Outbound-Query-Aggregate-Volume" } ],
[ "...", "RniId", "rni-xxxxxxxxxxxxx", { "id": "m2", "label": "RNI-us-east-1b-aggregate-volume" } ],
[ "...", "rni-xxxxxxxxxxxxxxx", { "id": "m3", "label": "RNI-us-east-1a-aggregate-volume" } ]
],
```

## 4. Suggest specific test cases

Samples:
#### Common use case: create a stack output or SSM param with value returned from !GetAtt {{resolverendpoint}}.RniIds

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

Samples:
#### Resolver Metric announcement at https://aws.amazon.com/about-aws/whats-new/2020/01/query-volume-metrics-now-available-for-amazon-route-53-resolver-endpoints/

## 6. Category

4. Networking & Content (VPC, Route53, API GW,...)

Start by reviewing the AWS::Route53Resolver::ResolverEndpoint documentation and the CloudFormation coverage implementation for resource attributes. Confirm how GetAtt attributes are represented and tested, then add RniIds so it returns the current resolver network interface IDs and supports the stack output or SSM parameter use case described here.

Written by the indexing model from the issue text.