refactor: use official ollama sdk (#744)

examples/ollama.ts

packages/core/e2e/fixtures/llm/anthropic.ts

packages/core/e2e/fixtures/llm/openai.ts

packages/core/package.json

packages/core/src/ChatHistory.ts

packages/core/src/QuestionGenerator.ts

packages/core/src/ServiceContext.ts

packages/core/src/Settings.ts

packages/core/src/agent/openai.ts

packages/core/src/embeddings/OpenAIEmbedding.ts

packages/core/src/index.ts

packages/core/src/llm/fireworks.ts

packages/core/src/llm/groq.ts

packages/core/src/llm/index.ts

packages/core/src/llm/ollama.ts

packages/core/src/llm/openai.ts

packages/core/src/llm/together.ts

packages/core/tests/CallbackManager.test.ts

packages/core/tests/MetadataExtractors.test.ts

packages/core/tests/utility/mockOpenAI.ts