Add request & response hooks + example

[zarqman]

Exploratory PR to add request & response hooks for the primary purpose of configuring multiple timeouts. Potentially useful for metrics or other things too.

Part 1/2: socketry/protocol-http1#21 Part 2/2: this PR

See #127 for discussion.

Types of Changes

• New feature.

Contribution

• [ ] I added tests for my changes.
• [ ] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.