No way to reject a route

### Describe the bug
Let's say I want to have an SPA served from https://example.com, and its APIs from https://example.com/api/name-of-api.

I would imagine setting up the two routes like this:

```json
[
{
"RouteId": "app",
"Match": {
"Path": "/{**any}"
},
"ClusterId": "app"
},
{
"RouteId": "first-api",
"Match": {
"Path": "/api/first-api/{*any}"
},
"ClusterId": "first-api"
},
{
"RouteId": "second-api",
"Match": {
"Path": "/api/second-api/{*any}"
},
"ClusterId": "second-api"
}
]
```

All three of these URLs will work as intended:
https://example.com/
https://example.com/api/first-api
https://example.com/api/second-api

But this URL will also be accepted, and quietly return the same as https://example.com
https://example.com/api/third-and-totally-invalid-api

Ideally, I would want to instruct the proxy to reject that route with a 404, perhaps with something like this:

```typescript
{
"RouteId": "invalid-api",
"Match": {
"Path": "/api/{*any}"
},
StatusCode: 404
},
```

### To Reproduce
Listed in the description

### Further technical details

Yarp.ReverseProxy 1.1.1
On Windows 10, .NET 6