Add policy item of retaining only specified latest versions of lambdas

### Describe your idea/feature/enhancement

* the SAM template should have an item policy on how many versions of the lambda should be retained (last versions). Could be further feature rich like keeping only specific versions.

### Proposal

* Adding section in the `Resources` should be appropriate way to go imho. Something like:
```
Resources:
RetainVersions:4
```

### Additional Details
This would help save a lot space for large companies who does not use
some abstraction on top of lambdas.