Cannot define a non-JSON-decoded requestBody

Version tested: sbt-guardrail 0.52.1

Using the OpenAPI 3 specification, I cannot define a requestBody which is interpreted as a plain string, or binary octet-stream. (Using the akka-http generator)

Using an example from the OpenAPI 3 docs, I should be able to do:
```
requestBody:
required: true
content:
"text/plain":
```

However that generates an exception.

I can do this and guardrail compiles:

```
requestBody:
required: true
content:
"text/plain":
schema:
type: string
format: string
```

But then in the generated Routes.scala, I still see json unmarshalling via Circe being attempted.