Request to add redis cache function

[liang-hiwin]

At present, unbound has supported the backend database as a secondary cache in the previous versions. When Unbound cannot find an answer to a query in its built-in in-memory cache, it consults the specified backend. If it finds a valid answer in the backend, Unbound uses it to respond to the query without performing iterative DNS resolution. If Unbound cannot even find an answer in the backend, it resolves the query as usual, and stores the answer in the backend.