Is your feature request related to a problem? Please describe.
Similar to having ability to create a new session, it'll be helpful to create artifacts using FASTApi. The use-case is simple -- if the client has to upload an artifact before making an LLM request, this api will allow one to do that especially while using in-memory artifact service.

Describe the solution you'd like
A post endpoint to create a new artifact given app_id, user_id, session_id

Describe alternatives you've considered
An alternative is to use GCSArtifactService and upload directly to GCS but there's no alternatives to uploading to InMemoryArtifactService as far as I know.

Additional context
Nope.