Refine `Content-Type` negotiation

The `parseBody` function is only using `startsWith` and `endsWith` to figure the content type.
https://github.com/apollographql/datasource-rest/blob/5ac9b524b1227b018c522da1a4e53425d2b86024/src/RESTDataSource.ts#L288-L289

In most cases (`application/json`) this is fine. But for users depending on the `+json` condition, this breaks for cases like this one:
```
application/ld+json; charset=utf-8
```

Ref: https://github.com/apollographql/datasource-rest/discussions/228#discussioncomment-6401539

Start in src/RESTDataSource.ts at the parseBody logic around lines 288-289. Reproduce negotiation with application/ld+json; charset=utf-8 and compare it with application/json. Done means +json content types with parameters are recognized correctly without breaking the existing application/json case.

Written by the indexing model from the issue text.