Fix perplexity baseline to actually search the web

[nbalepur]

Adding logic so the perplexity baseline can search the web, namely by:

1. Setting search_mode parameter to "" by default
2. Adding reasoning_effort as another parameter in the solver
3. Making an extra solver between Perplexity's and Gemini's formatted_solver that adds the references to the end of the prompt (called add_perplexity_references)

With this repo, I re-ran the scores for SQA and Litqa2, respectively:

Which are quite similar to what I shared earlier on slack for these two benchmarks (some variance but nothing crazy imo):

[amanpreet692]

LGTM!