[srp-server] refine build flags for SRP server and Advertising Proxy

[superwhd]

This PR introduces a macro OTBR_ENABLE_SRP_SERVER to indicate whether the SRP server feature is enabled in OT. In this way we can use this macro instead of OTBR_ENABLE_SRP_ADVERTISING_PROXY to guard the code block that depends on the otSrpServer* API.

This PR also introduces a CMake option OTBR_OT_SRP_ADV_PROXY to configure whether to use the OT core Advertising Proxy.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.18%. Comparing base (2b41187) to head (feeb501).
Report is 984 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2763       +/-   ##
===========================================
- Coverage   55.77%   44.18%   -11.60%     
===========================================
  Files          87      113       +26     
  Lines        6890    13588     +6698     
  Branches        0      965      +965     
===========================================
+ Hits         3843     6004     +2161     
- Misses       3047     7266     +4219     
- Partials        0      318      +318     

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[Irving-cl]

LGTM 👍