docs(examples): add service consumer/provider HTTP example

[adityagiri3600]

✈️ Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

📝 Summary

Adds a new HTTP example for a service that is both consumer and provider.

• adds examples/http/service_consumer_provider/
• updates examples/http/README.md and examples/README.md

🔥 Motivation

Closes #1274.

🔨 Test Plan

uv run ruff check examples/http/service_consumer_provider
cd examples/http/service_consumer_provider && uv run --group test pytest
cd /root/oss/pact-python && uv run mkdocs build --strict

🔗 Related issues/PRs

• Closes Add a complete example with a consumer/provider service #1274
• Related: Post v3 tracking issue #1272, docs(examples a->b->c): starting point of implementing Pact tests where there is a chain of services #276