feat(volo-http): add SSE client support with Accept header validation for server side

[Joshuahoky]

Motivation

SSE (Server-Sent Events) support was missing a client-side implementation for parsing SSE streams, and the server-side had no validation of the Accept header.

Solution

Added SseReader for parsing SSE streams from server responses
Added SseExt extension trait on Response for converting a response into an SseReader via into_sse()
Added ResponseExt trait with is_sse() for checking if a response has Content-Type: text/event-stream
Added Accept header validation in Endpoint::call
Added client and server examples demonstrating SSE usage

[codecov]

Codecov Report

❌ Patch coverage is 95.14286% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.61%. Comparing base (6220d5b) to head (ce0c727).

Files with missing lines	Patch %	Lines
volo-http/src/server/response/sse.rs	0.00%	7 Missing ⚠️
volo-http/src/client/sse.rs	98.17%	6 Missing ⚠️
volo-http/src/server/route/router.rs	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #643      +/-   ##
==========================================
+ Coverage   43.72%   44.61%   +0.89%     
==========================================
  Files         161      162       +1     
  Lines       19679    20028     +349     
==========================================
+ Hits         8604     8936     +332     
- Misses      11075    11092      +17     

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