[test] add unit test framework for ThreadHost APIs (RCP)

[Irving-cl]

This PR adds a unit test framework for testing ThreadHost APIs (RCP).

It's implemeneted by building rcp_host.cpp with FakePlatform in OT. A few small changes are done to make this work:

• implement openthread posix system APIs in fake_posix_platform.cpp. (like otSysInit, otSysMainloopProcess) ot::FakePlatform doesn't include these things and I think it's better to put it here.
• Remove the fixed build dependency of otbr-common on openthread-posix so that the unit test executable can link to the fake system API implementation instead those in openthread-posix.
• Move ThreadHost::ConvertToOtLogLevel to logging.hpp. Because otherwise we have to build thread_host.cpp into the test executable and then have more dependencies on NcpHost.

This PR also implements the first unit test case to test the RcpHost::SetThreadEnabled method.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 65.90909% with 30 lines in your changes missing coverage. Please review.

Project coverage is 45.78%. Comparing base (2b41187) to head (f9f1517). Report is 839 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/gtest/test_rcp_host_api.cpp	78.18%	10 Missing and 2 partials :warning:
src/common/logging.cpp	50.00%	9 Missing :warning:
tests/gtest/fake_posix_platform.cpp	40.00%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2549 +/- ## ========================================== - Coverage 55.77% 45.78% -9.99% ========================================== Files 87 104 +17 Lines 6890 12274 +5384 Branches 0 895 +895 ========================================== + Hits 3843 5620 +1777 - Misses 3047 6346 +3299 - Partials 0 308 +308 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.