[border-agent] add _meshcop-e service for ePSKc mode

[mia1yang]

Meshcop-e service is published when receiving DNS query for PTR records and BA is in the "ba ephemeralkey" active mode with the same instance name with Meshcop service and different UDP port number for the DTLS connection using ePSKc.
Test:

1. Active ePSKc mode using "ba ephemeralkey" commands, verified Meshcop-e package and service
2. DTLS connection could successfully set using ePSKc mode with Candidate Commissioner.
3. DTLS connection using ePSKc by inputing wrong ePSK in Candidate Commissioner, Meshcop-e service has been unpublished and no negative impacted with Meshcop service.
4. Meshcop-e service unpublished after timeout.

[codecov]

Codecov Report

Attention: Patch coverage is 4.00000% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 39.77%. Comparing base (2b41187) to head (755264c).
Report is 656 commits behind head on main.

Files	Patch %	Lines
src/border_agent/border_agent.cpp	4.00%	24 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2259       +/-   ##
===========================================
- Coverage   55.77%   39.77%   -16.01%     
===========================================
  Files          87       88        +1     
  Lines        6890     9778     +2888     
  Branches        0      721      +721     
===========================================
+ Hits         3843     3889       +46     
- Misses       3047     5691     +2644     
- Partials        0      198      +198     

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[wgtdkp]

LGTM 👍

[superwhd]

LGTM 👍

[abtink]

LGTM.