[FEATURE] Allow request-level customization of system and user prompts used in RAG

[austintlee]

Is your feature request related to a problem?
Today, prompts to LLMs are fixed at the (search) pipeline level. Sometimes, especially during active development, we would like to be able to try a number of different prompts both as system prompts and user prompts.

What solution would you like?
Allow search requests to take system and user prompts.

What alternatives have you considered?
None.

Do you have any additional context?
Add any other context or screenshots about the feature request here.

[austintlee]

I will create a new PR from austintlee#1 once #2213 is pushed.

[austintlee]

Merged in 2.13.