az role assignment create is NOT idempotent

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

**Command Name**
`az synapse role assignment create`

**Errors:**
```
(RoleAssignmentAlreadyExists) Role Assignment with roleId '6e4bf58a-b8e1-4cc3-bbf9-d73143322b78', principalId '1db446b3-dc3e-4eb5-b703-253431f879af' and scope 'workspaces/syntestadmin1982' within workspace 'syntestadmin1982' already exists.
Code: RoleAssignmentAlreadyExists
Message: Role Assignment with roleId '6e4bf58a-b8e1-4cc3-bbf9-d73143322b78', principalId '1db446b3-dc3e-4eb5-b703-253431f879af' and scope 'workspaces/syntestadmin1982' within workspace 'syntestadmin1982' already exists.
```

## To Reproduce:
Run the role assignment command twice. First time it will succeed if this role wasn't assigned before, second time will fail because it already exists.

- _Put any pre-requisite steps here..._
- `az synapse role assignment create --workspace-name {} --role {} --assignee {}`

## Expected Behavior
Not to fail but return the current assignment info. Similar to az role assignment to be idempotent to be used in automation.

## Environment Summary
```
Windows-10-10.0.19044-SP0
Python 3.10.10
Installer: MSI

azure-cli 2.46.0

Extensions:
bastion 0.2.1

Dependencies:
msal 1.20.0
azure-mgmt-resource 21.1.0b1
```
## Additional Context