[border agent] refactor dbus meshcop txt update

[Irving-cl]

This PR refactors the MeshCoP TXT update method in BorderAgent.

This is a DBUS method for vendors to update their vendor MeshCoP TXT values. The current call stack is:

DBus Handler -> ThreadHelper MeshCoP update callback -> BorderAgent Handle MeshCoP update

ThreadHelper is totally unnecessary here. So the new stack is simpler:

DBus Handler -> BorderAgent Handle MeshCoP update

Another intention is to unbind the BorderAgent module from ThreadHelper so that it can works in NCP design.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 43.77%. Comparing base (2b41187) to head (44bd6d2).
Report is 978 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dbus/server/dbus_thread_object_rcp.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2756       +/-   ##
===========================================
- Coverage   55.77%   43.77%   -12.00%     
===========================================
  Files          87      110       +23     
  Lines        6890    13411     +6521     
  Branches        0      956      +956     
===========================================
+ Hits         3843     5871     +2028     
- Misses       3047     7230     +4183     
- Partials        0      310      +310     

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[superwhd]

LGTM 👍

Good catch!