.Net: ADR for RAG pattern in SK

[dmytrostruk]

Resolves: #4578
Resolves: #3455

1. Added ADR which describes different approaches how to use RAG pattern in current version of Semantic Kernel and a couple of new possible approaches, which could be added in the future.
2. Added examples how to use RAG pattern with ChatGPT Retrieval Plugin, TextMemoryPlugin and custom plugin.
3. Small fixes to TextMemoryPlugin.

[roji]

Great write-up, posted some comments/thoughts below.

[stephentoub]

Otherwise, I think it's not a bad story to have the abstraction as a way to get up and running really quickly - without having to mess around with a custom plugin and understanding the specific vector DB's SDK; then, as new requirements come in, some users may "graduate" out of that story to interact directly with the vector DB, if they really need to.

I agree. There's significant value to having the abstraction, and we'd be doing folks a disservice by getting rid of it even if you can't do everything with it. You can do a lot with it (and we should ensure it's as good / full-featured as possible), including bootstrapping / getting up and running, and in a way that lets you postpone choice, i.e. you can write code without committing to a specific db, you can experiment, you can get your feet under you, and then if you later find you need to go beyond what the abstraction offers, you can break glass and use direct functionality on a specific db you commit to at that point.

[dmytrostruk]

@roji @stephentoub Thank you for feedback, it's very helpful! We will take it into account when deciding on next steps.