✨ [New]: Cache secrets on a more fine grained level

**Is your feature request related to a problem? Please describe.**
When registering a KeyVault secret-provider, we can specify a cache-duration. The drawback of this approach is that this cache-duration applies for all secrets that are being retrieved.
I think it would be good to have the ability to specify the cache-duration on a 'per secret' level.

**Describe the solution you'd like**
Provide an overload of the `GetSecret` method that takes a `TimeSpan` that defines the cache-duration for the secret that is being retrieved.