s2n-quic-events: Generate C bindings to publish events

[goatgoose]

Problem:

Related to #2764

s2n-quic-events currently generates a publisher interface in Rust, allowing events to be published from within s2n-quic. However, s2n-tls is written in C, so events will need to be published with C APIs. C bindings will be created for this purpose.

Out of scope:

For now, we intend to allow events to be subscribed to only from Rust, so we don't need to expose the subscriber interface to C yet.