Use different HTTP clients based on providers

[Kludex]

This PR solves the simple test case:

from pydantic_ai import Agent

openai_agent = Agent('openai:gpt-4o-2024-11-20')
vertex_agent = Agent('google-vertex:gemini-1.5-pro')

result = openai_agent.run_sync('Generate a random number between 1 and 100')
print(result.data)

result = vertex_agent.run_sync('Generate a random number between 1 and 100')
print(result.data)

I'm not able to create proper tests with VCR. We need to check that.

---

This PR uses the same underlying transport, which means the connection pool is shared between providers, but not the client itself - where we do auth, and set the base_url, etc.

This PR also uses a single HTTP client for downloading external resources (e.g. images and documents).

---

• Closes 401 when running Gemini on VertexAI in the Cloud #1186
• Closes Getting 401 request error with open ai service account keys #1221

[github-actions]

Docs Preview

commit:	289da7b
Preview URL:	https://a3f36704-pydantic-ai-previews.pydantic.workers.dev