Add JSON-RPC notifications

[BenediktBurger]

According to JSON-RPC specs, a server MUST NOT respond to a notification (a message without an id).

• This PR fixes that PyLECO did not adhere to that specification.
• Allow to send notifications: Follow up in Add easy interface for sending notifications like ask_rpc #132

Supports development of #91 due to notifications

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.09%. Comparing base (25ae333) to head (a73be50).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
+ Coverage   91.04%   91.09%   +0.04%     
==========================================
  Files          36       36              
  Lines        2905     2919      +14     
  Branches      293      298       +5     
==========================================
+ Hits         2645     2659      +14     
  Misses        224      224              
  Partials       36       36              

Flag	Coverage Δ
unittests	91.09% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.