Provide a service method to list existing indexes in the Semantic Memory

[kheiman61]

It's ok for now that if I provide an index that doesn't exist (when uploading a file) it creates the index, but if I want to create an 'admin' app that lets me see and maintain my indexes I have to hardcode my valid index names somewhere so I can see/select them in my app, or roll my own code in the dotnet service to list the indexes that currently exist in whatever vector db I am set up to use.

For the record I really like this project and looking forward to its rapid growth - it really makes it simple to implement low cost POC's to demonstrate value as we move towards more investment in AI.