feat: added Gemini tool calling support (#973)

.changeset/fuzzy-tigers-accept.md

apps/docs/docs/examples/agent_gemini.mdx

apps/docs/docs/modules/agent/index.md

examples/gemini/agent.ts

packages/llamaindex/src/agent/utils.ts

packages/llamaindex/src/llm/gemini/base.ts

packages/llamaindex/src/llm/gemini/types.ts

packages/llamaindex/src/llm/gemini/utils.ts

packages/llamaindex/src/llm/gemini/vertex.ts