[Test-Proxy] Enforce `content-type` consistently across Various API Controllers

Right now, when one `POSTS` to `Record/Stop` the test-proxy accepts a JSON body that is essentially a simple dictionary.

```json
{
"variable": "value"
}
```

We do this as part of [Variables](https://github.com/Azure/azure-sdk-tools/tree/main/tools/test-proxy/Azure.Sdk.Tools.TestProxy#storing-variables) support.

There's a problem though. When POSTING to `Record/Stop/` with a body to configure a sanitizer, `Content-Type: Application/Json` is **required**. That in and of itself is not the problem.

The real problem is that, when originally implementing the `Admin` controller, I was quite a newbie and did _not_ mandate the presence of `content-type: application/json` header when POSTING to

- `/Admin/AddSanitizer`
- `/Admin/AddTransform`
- `/Admin/SetMatcher`

Even though we explicitly look at the json-converted http body to retrieve the _settings_ of the sanitizer (targeting is in headers).

For consistency's sake, we absolutely should do the right thing and require the content-type wherever a JSON body is posted.

Mandating the presence of this content-type is technically a breaking change, but I'm going to pull the bandaid off this week rather than later.