Support creating IdentityStore / SSO Users (AWS::IdentityStore::User)

### Name of the resource

Other

### Resource name

AWS::IdentityStore::User

### Description

I'd like to be able to manage Identity Center users via CloudFormation. I think you'd need to use the following APIs:

https://docs.aws.amazon.com/singlesignon/latest/IdentityStoreAPIReference/API_CreateUser.html
https://docs.aws.amazon.com/singlesignon/latest/IdentityStoreAPIReference/API_UpdateUser.html
https://docs.aws.amazon.com/singlesignon/latest/IdentityStoreAPIReference/API_DeleteUser.html

### Other Details

_No response_

Start by reviewing the CloudFormation coverage-roadmap issue and the linked Identity Store API references for CreateUser, UpdateUser, and DeleteUser. Determine the repository entry point and contribution process for adding AWS::IdentityStore::User coverage. Done means CloudFormation can manage Identity Center users through the requested create, update, and delete operations.

Written by the indexing model from the issue text.