Fetches a value from the store given its key.
The key of the value to retrieve.
A promise that resolves with the value associated with the key, or undefined
if no value exists for that key.
Sets the value for a key in the store.
The key under which to store the value.
The value to be stored.
A promise that resolves when the value has been set.
Interface for cache implementations used by to store resolved DID documents.