Get secret with specific version ID

Currently, the only way to retrieve a secret is by its ID with `SecretCache.getSecretString(secretId)`. There is no way to get a specific _version_ of a secret using its ID and versionId. One can build a `SecretCacheConfiguration` object and call `setVersionStage`, then pass that configuration to a `SecretCache` constructor; however, this still only allows retrieving a specific version of a secret by its Id and _stage_, and would require using the same stage for all secrets (unless constructing separate caches). It would be nice if `getSecretString` took an optional `versionId` string, so that we could use specific versionIds and not just version stages.