Stub out EventV0 specs so bindings still generate

[Shaptic]

What

Adds a stub to the EventV0 branch of ScSpecEntry.

Why

Bindings fail to generate if your wasm uses the new #[contractevent] macro, but they shouldn't:

$ stellar contract bindings typescript --output-dir testing --wasm target/wasm32v1-none/release/events.wasm 
ℹ️ Loading contract spec from file...

thread 'main' panicked at /home/george/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/soroban-spec-typescript-23.0.0/src/types.rs:255:40:
not yet implemented: EventV0 is not implemented yet

We should still let them be generated, even though there aren't bindings for the events themselves.

Known limitations

n/a, I confirmed that building this branch and running the above commands generates bindings.