Add client handshake example

The current model in tests/client.c has a workflow that looks like:

1. Create a rustls_connection (which does not involve reading or writing from the socket)
2. Write unencrypted bytes to the connection
3. In a for loop, alternate writes and reads based on the socket state and connection state. The handshake is performed implicitly, followed by the write of the unencrypted bytes.

Some of the C libraries I've seen have a different workflow - they have API's that look like

1. Socket connection and TLS handshake (cr_connect_nonblocking in curl)
2. API for writing data (cr_send in curl, pgtls_write in Postgres libpq/fe-secure.c)
3. API for reading data (cr_recv in curl, pgtls_read in Postgres)

It might be helpful to add a second example, or break out the logic in the current example so the handshake is performed separately. Especially since as implemented in vtls/rustls.c, the logic to perform a handshake is not obvious (call the write method with an empty buffer), and there's no explicit "handshake" method in crustls.h

Even if it's the same workflow under the hood, conceptually this might help when porting code that currently exists in C.

Start with the workflow in tests/client.c, then read crustls.h and the handshake-related logic referenced in vtls/rustls.c. Add a second example or refactor the existing example to show the handshake separately, and confirm that the resulting workflow is clear for code ported from other C TLS libraries.