Allow fetch and creation of RSA certificate files for Vault TLS secrets

[ozozgun]

Currently the plugin can output only .env file that contains key-value pairs.
If we want to use the plugin to fetch a certificate from the Vault KV2 store, and output it as a certificate file, so we can upload it to kube, we cannot do it.
We need to create an output type that can write plain text file instead of .env format, so we can use it to output an RSA key.