feat: implement context-aware agent functionality. (#1394)

.changeset/tough-oranges-itch.md

apps/docs/docs/examples/context_aware_agent.mdx

apps/docs/docs/guides/agents/4_agentic_rag.mdx

examples/contextAwareAgent.js

packages/core/src/agent/index.ts

packages/llamaindex/src/agent/anthropic.ts

packages/llamaindex/src/agent/contextAwareMixin.ts

packages/llamaindex/src/agent/index.ts

packages/llamaindex/src/agent/openai.ts