Consolidate various "evals" testapps

[ssbushi]

In this PR:

• Combine cat-eval and eval testapp to evals. This new testapp will be the comprehensive home for eval features.
• Rename byo-eval to custom-evaluators
• Delete evaluators-gut-check
• Tone down on eval focus in rag testapp. rag will have the default genkitEval plugin install to let users dabble with evaluations, but we move other eval plugins / focus into evals testpp.`
• rag now hosts the pdfQAFirebase flows which showcases using Firestore vector store for RAG.