feat(bindings/s2n-tls): add client_hello_version

[jmayclin]

Description of changes:

This adds bindings for the s2n_connection_get_client_hello_version method.

Testing:

Added simple test as a sanity check to the rust bindings. Functionality testing is contained in the C codebase.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[goatgoose]

Why is s2n_connection_get_client_hello_version() needed instead of s2n_connection_get_client_protocol_version() or s2n_client_hello_get_legacy_protocol_version()?

[lrstewart]

The fact that this question was asked probably means we need a comment.

I added a comment to s2n_client_hello_get_raw_message about this use case, but it isn't mentioned on s2n_connection_get_client_protocol_version itself. The usage guide also has a section on the different protocol versions, but s2n_connection_get_client_protocol_version isn't even mentioned. Clearly we've got some documentation gaps :(

And of course, updating the C documentation doesn't fix Rust :( #4615

[lrstewart]

Oh wait, there is a note elsewhere in the usage guide about it:

s2n-tls/docs/usage-guide/topics/ch10-client-hello.md

Lines 34 to 37 in 933f379

	You can determine whether an SSLv2 ClientHello was received by checking the value
	of `s2n_connection_get_client_hello_version()`. If an SSLv2 ClientHello was
	received, then `s2n_connection_get_client_protocol_version()` will still report
	the real protocol version requested by the client.

[goatgoose]

Ohh got it okay, that makes sense. Would this be easier to understand if we just made the rust version be client_hello_is_sslv2() rather than expose an additional protocol version getter?