Pipelines block inputs for Guardrail hooks

[lukehinds]

Ability to gate and modify inputs / outputs:

1. Given an input, apply logic around block or allow (provide an interface, single method).
2. Given an input, add a system message (logic to layer in system message, according to the event nature)

We will use LiteLLM hooks for now, but its a bit messy and should be placed into tech debt.

[jhrozek]

1 (Given an input, apply logic around block or allow (provide an interface, single method)) is part of PR #91

[jhrozek]

I will work on 2. after fixing up the provider interfaces (#94)

[lukehinds]

Jakub will break into two mediums to cover system prompt inclusion.

[jhrozek]

I broke off the system prompt inclusion into #103

[jhrozek]

This was done in #91