Add Multi-Evaluator Node

[ianarawjo]

Currently each evaluator is a single node. Although code evaluators can support dictionary outputs for multiple metrics, this behavior is relatively obscure and only works for code-based assertions. To better support "iterative refinement" of prompts for developers, we should make it easier to add multiple, independent evaluations in the same node (i.e. a mix of named code assertions and LLM scoring prompts).

To implement this we need to:

• abstract out the code eval and LLM scorer subcomponents from their respective nodes, so that they can be added independent of the node (like how the Response Inspector view works)
• (possibly useful, but not strictly required) make LLM scorer nodes locked to true/false values by default (?), or otherwise re-think them to be easier to write and add expected output types that scores stick to