Default Service Mesh to Enabled

#### Feature Description

Currently with default non-dev settings Consul has the Connect feature disabled. This means that the servers do not initialize the CA, the various connect RPCs/HTTP APIs will return errors, agents will not listen on the gRPC port for xDS and that agents will not be performing central service configuration. As service mesh is a core use case of Consul the idea is that it should be enabled by default.

New server defaults:

```
connect {
enabled = true
}
```

New agent defaults:

```
enable_central_service_config = true

ports {
grpc = 8502
}
```

#### Considerations

While this isn't technically a breaking change in functionality we should still treat it as such and save it for a major release. Many users would not want an upgrade to start opening new ports or putting more stress on their servers (which the central service config bit can do). So while it would be good to default these to on for the day 0-1 UX of trying out Consul service mesh we need to very loudly advertise that these changes are happening and provide clear/simple instructions on how to opt-out for existing users or new users not utilizing Consul as a service mesh.