Policy rules with same index value can be assigned to same policy

[collin-wicker]

Environment

• Python version: 3.9.18
• Nautobot version: 2.2.0b1
• nautobot-firewall-models version: 2.0.3

Expected Behavior

Expected a warning message that explains that a policy is unable to contain two policy rules with the exact same index value.

Observed Behavior

The policy rules with the same index values were permitted to be added to the same policy.

Steps to Reproduce

1. Create a policy
2. Create a policy rule with index value of 10 and assign it to the policy that was just created
3. Create a second policy rule with an index value of 10 and assign it to the policy that was just created