Add built-in features for server-sent events

[RobertoPrevato]

• Add built-in features for Server-Sent events.
• Add a function to detect when the server process is terminating because it received a SIGINT or a SIGTERM command.
• Add support for request handler normalization for methods defined as asynchronous generators.

[codecov-commenter]

Codecov Report

Attention: 69 lines in your changes are missing coverage. Please review.

Comparison is base (4fc7a4b) 97.48% compared to head (185bd1d) 96.46%.

Files	Patch %	Lines
blacksheep/server/normalization.py	33.78%	49 Missing :warning:
blacksheep/server/process.py	35.29%	11 Missing :warning:
blacksheep/server/sse.py	60.00%	8 Missing :warning:
blacksheep/server/application.py	66.66%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #469 +/- ## ========================================== - Coverage 97.48% 96.46% -1.03% ========================================== Files 67 69 +2 Lines 6365 6477 +112 ========================================== + Hits 6205 6248 +43 - Misses 160 229 +69 ```

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