Implement a dry-run/check/plan mode

### Problem Statement

Hi.
We are considering using kecloak-config-cli to provision the huge amount of resources (realms, clients, scopes, flows…) in ou keycloak and would love to have a dry-run-like checking method.

So far we haven't found a smart way to see changes before they are actually applied.

### Proposed Solution

- implement a `--dry-run` flag

See this as an equivalent of:
- `ansible-playbook --check --diff`
- `terraform plan`

It shoud create a diff-like output showing changes (creates / deletes / updates) that should be performed

### Environment

- Keycloak Version: [26.1.5]
- keycloak-config-cli Version: [6.5.1]
- Java Version: [21]

### Additional information

_No response_

### Acceptance Criteria

As a user, the dry-run mode should display:
- changes that may occur without applying them if changes have been found
- no changes if current and desired state are the same (idempotence)