[application] simplify fdset update

[Irving-cl]

This PR adds methods to MainloopContext to encapsulate the process to add an fd to some fd sets of it and update the maxFd. This simplifies the implementation. Previous discussion for this change.

[codecov]

Codecov Report

Attention: Patch coverage is 73.07692% with 7 lines in your changes missing coverage. Please review.

Project coverage is 45.56%. Comparing base (2b41187) to head (eadd970).
Report is 788 commits behind head on main.

Files with missing lines	Patch %	Lines
src/common/mainloop.cpp	66.66%	2 Missing and 3 partials ⚠️
src/dbus/server/dbus_agent.cpp	75.00%	1 Missing ⚠️
src/utils/infra_link_selector.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2469       +/-   ##
===========================================
- Coverage   55.77%   45.56%   -10.22%     
===========================================
  Files          87       99       +12     
  Lines        6890    11663     +4773     
  Branches        0      849      +849     
===========================================
+ Hits         3843     5314     +1471     
- Misses       3047     6072     +3025     
- Partials        0      277      +277     

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[abtink]

Nice! :) Thanks @Irving-cl for adding this. 👍 👍