Add context manager support for bot client

[JrooTJunior]

The bot client now supports the context manager protocol, providing automatic resource management. This enhancement helps to automatically close the session when leaving the context, which cleans up resources better. The documentation and tests have been updated accordingly to illustrate this new feature. Moreover, an example of usage without a dispatcher has been provided to clarify its use in simple cases.

From now you can use:

async with Bot(...) as bot:
    ...

instead of

async with Bot(...).context():
    ...

[github-actions]

✔️ Changelog found.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (9756dac) to head (b2b2fcf).

❗ Current head b2b2fcf differs from pull request most recent head 5684bc3. Consider uploading reports for the commit 5684bc3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##           dev-3.x     #1468   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          422       422           
  Lines        10932     10937    +5     
=========================================
+ Hits         10932     10937    +5     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Files	Coverage Δ
aiogram/client/bot.py	100.00% <100.00%> (ø)