feat(queryEngineTool): add query engine tool to agents (#509)

.changeset/smart-ligers-occur.md

apps/docs/docs/modules/agent/query_engine_tool.mdx

examples/agent/query_openai_agent.ts

examples/subquestion.ts

packages/core/src/agent/runner/base.ts

packages/core/src/engines/query/SubQuestionQueryEngine.ts

packages/core/src/tools/QueryEngineTool.ts

packages/core/src/tools/index.ts

packages/core/src/types.ts