feat: Add Server-Sent Events transport for incremental delivery

[jwaldrip]

Summary

This PR adds Server-Sent Events (SSE) transport support to absinthe_plug for incremental delivery with @defer and @stream directives.

⚠️ DEPENDENCY NOTICE

This PR depends on gigsmart/absinthe#gigmart/defer-stream-incremental
The main absinthe package must be merged first before this PR can be merged.

Features

• Server-Sent Events (SSE) adapter for HTTP streaming
• Multipart response format support
• Proper chunked transfer encoding
• Graceful connection handling

Implementation

• Absinthe.Plug.Incremental.SSE - SSE transport adapter
• Event stream formatting
• Connection lifecycle management

Testing

• SSE protocol compliance
• Chunked encoding validation
• Connection recovery scenarios

Breaking Changes

None - incremental delivery is opt-in and backward compatible.