feat(example): add client 100 Continue with fixed body POST example

[heiwais25]

Notes

This commit adds a new client example demonstrating HTTP 100 Continue functionality.

Background

Hyper Client originally didn't have support for the HTTP 100 Continue functionality.

As per RFC 7231, the client is expected to wait for the 100 Continue from the server after sending only the header parts.

• Other Doc: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status/100

Following the discussion in #3833, the on_informational hook has been officially exposed, allowing the Hyper client to support 100 Continue functionality based on its usage.

This example aims to help new developers easily implement 100 Continue functionality with the Hyper client. The included server is intended only to provide a complete, working scenario.

Tests

Since it is not easy to test 100 Continue with public APIs, this was tested in the following scenario using the included mock server.

Run mock TCP server:

cargo run --example client_100_continue --features="full" -- --server 8080

Run client:

cargo run --example client_100_continue --features="full" -- http://127.0.0.1:8080