Allow passing custom Last-Event-Id

Add an option `lastEventId` to `EventStreamOptions`, so the application can pass a custom last event ID.

This is important to have, so that a user can specify a last event ID per query parameter or similar, since the native DOM `EventSource` does not allow customizing the `Last-Event-Id` header.

Use case: The user wants to stream data from a certain point onwards (e.g. the last time they were online) when opening the stream. The `Last-Event-Id` header is only automatically set when reconnecting an already open stream that had some connectivity issue.