Autogenerated enum conflicts with existing enum

# What

When a `schema.json` defines something like

```
"kind": "ENUM",
"name": "Role",
"description": null,
"fields": [],
"inputFields": [],
"interfaces": [],
"enumValues": [
...
]
```

the resulting Swift code looks like
```
public enum Role: RawRepresentable, Equatable, Apollo.JSONDecodable, Apollo.JSONEncodable {
...
}
```
When used in an app that's been around for a while, it's likely an enum with the same name will already be defined, so the two will conflict.

# Current Solutions

There are now a couple paths forward:

1. Rename `Role` on the server.
1. Rename the existing `Role` enum on the client.

Neither are ideal of course.

# Proposed Solutions

I think a reasonably clean solution would be to provide something similar to `--customScalarsPrefix`, but where it prefixes enums. In my personal scenario, I would simply provide `"Apollo"` for that config, giving me `public enum ApolloRole`, which I could then map to my existing role if needed.

- [x] feature

Start by tracing how schema.json enum definitions become generated Swift enums, using the existing customScalarsPrefix configuration as the closest reference. Done means a configuration can prefix generated enum names, producing output such as ApolloRole without conflicting with an existing Role enum.

Written by the indexing model from the issue text.