Refactor SSE codec

[ryanolson]

          Can you add a `todo: consider refactor with https://github.com/launchdarkly/rust-eventsource-client/blob/main/eventsource-client/src/event_parser.rs` or something?

Originally posted by @rmccorm4 in #123 (comment)

[paulhendricks]

Dove into this a bit and wanted to capture some findings.

eventsource-client

• This is actually super promising and the internals of event_parser.rs look relevant.
• Uses SSE endpoint URL instead of some exposing some low-level Decoder API that operates on raw byte streams.
• Doesn't expose these internals for use as public.

async-sse

• Codebase hasn't been touched in >4 years.
• async-sse does not support comments but instead logs them while we capture comments. It's not clear if capturing comments is critical (we use them with event errors, see next bullet).
• Our codec checks if an event is an error and handles it using comments where as async-sse one does not.
• The SSE specification states that if no event name is provided, it should default to "message". In our codec, however, when dispatching an event the event field is set to None if the event type buffer is empty. It's not clear to me which behavior we prefer but I imagine we may want to be as close to the SSE spec as possible.

tl;dr There may be some sense in continuing to roll our own SSE codec here, especially if we want custom functionality. What common structs exist in the Rust ecosystem like async-sse do not appear to be currently maintained or like eventsource-client do not appear to expose internal primitives we can use.