Expose names of ports from StatelessDataflowDefinition and allow multiple extension directories

The StatelessDataflowDefinition class is generic, so it can potentially hold a flow that is comprise of a byte[], a VersionedFlowSnapshot, etc. But this makes it difficult to know the names of Input Ports & Output Ports. We currently expose the names of configured Failure Ports but should expose the names of all Input Ports and Output Ports that are a part of the dataflow, as well.

Additionally, in improving this configuration API, we should allow for multiple Extensions directories to be specified. Currently, we allow for the NAR directory to be specified, which is the location of the stateless nar itself and supporting libraries. We also expose a configuration option for an Extensions directory. But if there are missing extensions, the Extensions directory is used to write those extensions when they are downloaded. This means that we cannot mount a read-only volume that contains extensions. This would be helpful when running in some cloud environments where we may have access to multiple read-only directories.