Streamable HTTP resumability + redelivery + SSE polling via server-side disconnect

[MackinnonBuck]

Overview

This PR implements the following features:

• Client-side polling via the Last-Event-ID header
• An ISseEventStreamStore abstraction to allow for storage and replay of SSE events
• Server-side resumability of SSE streams via the Last-Event-ID header
• Server-side disconnect (to enable client-side polling)

Each of these features has been split into its own commit for ease of review.

Description

There are two disconnection scenarios covered by this PR:

• Network errors (unintentional disconnection)
• Server-side programmatic disconnection

If a network error occurs, and an ISseEventStreamStore is configured, then the client may attempt to reconnect by making a GET request with a Last-Event-ID header. The server will then replay stored events before continuing to use the new GET response to stream remaining events. If further disconnections happen, the client may continue to make new GET requests to resume the stream. This applies for both client-initiated requests (POST) and the unsolicited message stream (GET).

However, the server can also initiate a disconnection and force the client to poll for updates. This is useful for avoiding long-running streams. This can be done via the new RequestContext<TParams>.EnablePollingAsync(TimeSpan retryInterval, CancellationToken cancellationToken = default) API. When the client reconnects via GET with a Last-Event-ID, the response will only contain events currently available in the ISseEventStreamStore before completing. The client must continue initiating new GET requests at the specified retryInterval until the final response is received.

Fixes #510
Fixes #1020

[mikekistler]

I didn't get through all of this but want to share the comments I've made so far.

[halter73]

This looks really good. Thanks for responding to all the feedback!