Custom secretResolver should return unknown/any type?

[Eskibear]

Use KeyVaultOptions.secretResolver to resolve the secret reference with customized logic, e.g. parse JSON string into an object.

    /**
     * Specifies the callback used to resolve key vault references that have no applied SecretClient.
     * @param keyVaultReference The Key Vault reference to resolve.
     * @returns The secret value.
     */
    secretResolver?: (keyVaultReference: URL) => string | Promise<string>;

Note: Here I think secretResolver should return known type instead of string, as we allow values of the config map to be any type.

Originally posted by @Eskibear in #78 (comment)