Get secrets by version ID

Currently, the only way to retrieve a secret is by its Id with `SecretsManagerCache.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 set `VersionStage`, then pass that configuration to a `SecretsManagerCache` 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 SecretsManagerCache.GetSecretString and the SecretCacheConfiguration usage described in the issue. Determine how an optional versionId can be passed for a specific secret without changing the existing stage-based behavior. Done means callers can retrieve a secret by both secret Id and versionId, while existing calls continue to work.

Written by the indexing model from the issue text.