Add FHIR sources and LLM config to healthchain.yaml

[jenniferjiangkells]

Description

Extends the app config schema and CLI scaffolding to support declaring FHIR data sources and LLM providers directly in healthchain.yaml, reducing boilerplate in user app code.

Changes:

• AppConfig: adds SourceConfig (FHIR source with env-prefix-based credential loading) and LLMConfig (provider/model/max_tokens with to_langchain() helper)
• healthchain.yaml template: adds commented-out sources and llm sections so new projects see the config shape immediately
• cli.py: scaffold template updated to R4B imports; sandbox run resolves from_path from config data.patients_dir if not passed explicitly
• app.py gateway template: minor fix