[AWS::BedrockAgentCore::Runtime] - [Coverage] - Add WorkloadIdentity property to link custom Workload Identities to a Runtime

### Name of the resource

Other

### Resource name

AWS::BedrockAgentCore::Runtime

### Description

When creating a Runtime resource using CloudFormation, we would like to create a custom WorkloadIdentity resource attach to it, so it is possible to control its name for permission actions and code calls. If it is not linked to the Runtime resource, it is not possible to activate neither application logging or traces or to use it in the code itself, for calls like get_workload_access_token.

In the documentation, it shows there is a WorkloadIdentityDetails associate to the resource but it is only a read-only parameter, and so it is not possible to get it as an output or set it.
https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/aws-properties-bedrockagentcore-runtime-workloadidentitydetails.html

### Other Details

_No response_

Start with the AWS::BedrockAgentCore::Runtime resource and the linked WorkloadIdentityDetails documentation to understand the existing read-only association. Confirm how a writable WorkloadIdentity property should link a custom WorkloadIdentity and what output or behavior should demonstrate completion.

Written by the indexing model from the issue text.