OptionalcapabilitiesCapabilities of the event sources this factory produces.
The SDK checks these before using a feature that not every transport supports -- for instance,
it only issues a POST or REPORT streaming request when customMethod is true. When this is
omitted, every capability is treated as unsupported.
Create an EventSource connected to the given URL with the given initialization options.
Factory for the EventSource implementation a client-side SDK uses for its streaming connection.
This should only be used when customizing the streaming transport. Typical usage of the SDK does not require implementing this.