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.

Start by tracing SecretCache.getSecretString(secretId) and the SecretCacheConfiguration.setVersionStage path named in the issue. Check how the cache constructor retrieves versions, then verify that an optional versionId can select one secret version independently of shared stages.

Written by the indexing model from the issue text.