feat: add Oracle OCI Generative AI provider

[luigisaetta]

Summary

Adds OCI Generative AI support through OCI's OpenAI-compatible Chat Completions endpoint.

• Adds the oci provider with API-key and Resource Principal authentication.
• Supports synchronous, asynchronous, and streaming chat completions.
• Adds the oci optional dependency extra.
• Adds OCI configuration and usage documentation.
• Adds unit tests and opt-in live integration tests for openai.gpt-5.5.

Usage

import aisuite as ai

client = ai.Client()
response = client.chat.completions.create(
    model="oci:openai.gpt-5.5",
    messages=[{"role": "user", "content": "Hello"}],
)

[luigisaetta]

The implementation has been validated locally with pre-commit, the non-integration test suite, and opt-in OCI live integration tests. No GitHub Actions workflow files are modified. Could a maintainer please approve the workflow run?