Add timeout support

[Olegt0rr]

What do these changes do?

Provides a way to work around issue https://github.com/sysid/sse-starlette/issues/89

Are there changes in behavior for the user?

Added the ability to set a timeout for interaction with a connection.

For example, if a hung connection does not read ping messages, then after a while we can automatically disconnect it to free up resources.

Related issue number

https://github.com/sysid/sse-starlette/issues/89

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (2cd91ea) to head (26251da). Report is 17 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #469 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 4 4 Lines 475 501 +26 Branches 54 57 +3 ========================================= + Hits 475 501 +26 ```

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