Allow arg/env in userdata for host-containers

**What I'd like:**
I'd like to be able to more easily parameterize `env` and `args` when running containers (similar to how I might w/ docker/k8s) using bottlerocket user data. I see there is a similar feature here: https://github.com/bottlerocket-os/bottlerocket/pull/1244, but IIUC, it doesn't quite go far enough to cover my use case.

[settings.host-containers.my-container]
enabled = true
env.MY_ENV_VAR = "my env var's value"
args = ["list", "of", "strings"]

**Any alternatives you've considered:**
I might be able to do this with just
```
[settings.host-containers.admin]
enabled = true
user-data = "aGkgdGhlcmUKaG93IGFyZSB5b3UK"
```
but this is beyond my current knowledge of bottlerocket.