Strings as SSE event IDs

The current SSE API models event IDs as non-zero integers. This appears to be more restrictive than the SSE specification, which describes the value as an "event ID string". See https://html.spec.whatwg.org/dev/server-sent-events.html#the-last-event-id-header

The current API prevents valid SSE use cases such as:
- UUIDs
- Opaque cursor tokens
- Composite identifiers (e.g. `stream-123:456`)
- Explicitly using `0` as an event ID (useful when talking about offsets into a stream)

Would it be possible to add support for strings? This would more closely match both the SSE specification.

I'm happy to submit a PR if a project maintainer could advise an approach. My instinct is to add an `IDString string` field alongside `ID int` and mark `ID` as deprecated. The string value would take precedence over the integer value if both are present. If having two separate fields is not desirable, an as an alternative, we could relax `ID` type to `any` and perform stringification dynamically, but I'd argue the two-field approach is clearer.

cc @danilvpetrov