feat(elasticloadbalancingv2): minimum Loadbalancer Capacity Unit (LCU) reservation (#32382)

### Issue # (if applicable)

None

### Reason for this change

ELB now supports for reserving minimum LCUs for both Application and Network load balancers.

ALB: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/capacity-unit-reservation.html
NLB: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/capacity-unit-reservation.html

### Description of changes

- Add `minimumCapacityUnit` to `BaseLoadBalancerProps`
- Verify whether `minimumCapacityUnit` is a positive integer

### Description of how you validated changes

Add both unit and integ tests.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: badmintoncryer