Implement last-event-id header

[piranna]

Id is being send, but header is being ignored. Allow to optionally handled It, just only i'm not sure if as a TimeOut to keep messages in Memory as a caché, or as an uset provided función that fully handled them.

[piranna]

Maybe an object with a get and a set methods could work, so internally it can be implemented whatever it fits. The most simple case could be just a dict where keys are the IDs and values are the events themselves.

[piranna]

Ok, I've found that it's not strictly necesary to add the support at sse-starlette, but instead it's possible to do it at the ContentStream provided as argument of EventSourceResponse constructor, by passing the header as an argument of the iterator, as storing it before yiending the data for the ServerSentEvent object.