[ecs-patterns]: allow use of existing load balancer in ApplicationMultipleTargetGroupsEc2Service

## This is a :rocket: Feature Request

### Description

Would like to be able to use an existing load balancer with ApplicationMultipleTargetGroupsEc2Service.

Load balancer pricing includes a fee per hour of load balancer running, which currently means about $18 per month. Running many services, each with its own balancer, can quickly add up.

Additionally, creating a separate load balancer for each service means we can't run multiple services under the same DNS name. So it's not possible to call different microservices depending on the path, for example (probably a common pattern?). Or group multiple subdomains on the same load balancer.

### Use Case

```
const service1 = new ApplicationMultipleTargetGroupsEc2Service(this, "Service1",{
...
targetGroups: [
{
containerPort: 80,
listener: "listener",
**pathPattern: "service1",**
priority: 1,
protocol: Protocol.TCP,
},
});
const service2 = new ApplicationMultipleTargetGroupsEc2Service(this, "Service2",{
targetGroups: [
{
containerPort: 80,
listener: "listener",
**pathPattern: "service2",**
priority: 2,
protocol: Protocol.TCP,
},
});
```

I would like that that two gets deployed to (Use the same LoadBalancer, instead of creating one for Each Service):
- myalb.amazonaws.com/service1
- myalb.amazonaws.com/service2

### Proposed Solution

Not have a workaround. Would need to skip ECS Patterns for this.

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

---

Start at the ecs-patterns implementation of ApplicationMultipleTargetGroupsEc2Service and inspect how it creates its load balancer, listeners, and target groups. Review the related tests, if present, before determining how an existing load balancer and listener could be supplied. Done means multiple services can share one load balancer with distinct path patterns, priorities, and target groups, covered by tests.

Written by the indexing model from the issue text.