Support for SSE Routes

[Badgerati]

Support in Routes should be allowed to enable SSE, and only on GET Routes.

as an example, some Set-PodeSseConnection function could be called from an Add-PodeRoute scriptblock which would set appropriate the Content-Type = text/event-stream header, as well as Cache-Control = no-cache; Connection = keep-alive; and Access-Control-Allow-Origin = * (if needed). A ClientId would also be generated for the connection, and set back as X-Pode-ClientId similar to websockets. This function would also ensure that the Request Headers contain an Accept = text/event-stream header, and that the Method = GET, otherwise the call should fail.

The above function could also check for a Last-Event-Id header on the Request. A new $WebEvent.Sse property could be populated with such info.

Pode would then internal keep the inbound connection open, mapped against the Event Type and ClientId. A new Send-PodeSseMessage could be used to send messages from server-to-client(s), with optional ID/Event messages as well. Similar to websockets, these messages will be sent asynchronously, in case there are a lot of clients to sent messages to.

A Close-PodeSseConnection function could be used to close an open client connection, or all connections for an event type.

Optional: an Add-PodeSseRoute wrapper function for Add-PodeRoute, which will automatically deal the simple SSE Route configurations - ie: just Set-PodeSseConnection.

Docs:

• https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events
• https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events