## Objective
Add a way to stream BRP requests when the data changes.
## Solution
#### BRP Side (reusable for other transports)
Add a new method handler type that returns a optional value. This
handler is run in update and if a value is returned it will be sent on
the message channel. Custom watching handlers can be added with
`RemotePlugin::with_watching_method`.
#### HTTP Side
If a request comes in with `+watch` in the method, it will respond with
`text/event-stream` rather than a single response.
## Testing
I tested with the podman HTTP client. This client has good support for
SSE's if you want to test it too.
## Parts I want some opinions on
- For separating watching methods I chose to add a `+watch` suffix to
the end kind of like `content-type` headers. A get would be
`bevy/get+watch`.
- Should watching methods send an initial response with everything or
only respond when a change happens? Currently the later is what happens.
## Future work
- The `bevy/query` method would also benefit from this but that
condition will be quite complex so I will leave that to later.
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Co-authored-by: Zachary Harrold <zac@harrold.com.au>
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