useResponseCache: invalidate cache all at once

**Is your feature request related to a problem? Please describe.**

If you don't setup a ttl at first, you may endup with a never invalidated cache even after adding new cache restriction in the config.

**Describe the solution you'd like**

Some very basic solution using the inMemoryCache but we should be generalised (proposed by @EmrysMyrddin):

```
import { createInMemoryCache, Cache } from '@graphql-yoga/plugin-response-cache'

export function createCache(): Cache {
let cache = createInMemoryCache()
return {
get: (...args) => cache.get(...args),
set: (...args) => cache.set(...args),
invalidate: (...args) => cache.invalidate(...args),
purge: () => {
cache = createInMemoryCache()
},
}
}
```

**Describe alternatives you've considered**

A custom implementation

**Additional context**

/