[thread-host] implement schedule migration for rcp host

[Irving-cl]

This PR implements the ScheduleMigration method in RcpHost.

[codecov]

Codecov Report

Attention: Patch coverage is 78.84615% with 11 lines in your changes missing coverage. Please review.

Project coverage is 46.31%. Comparing base (2b41187) to head (05bfefb).
Report is 852 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ncp/rcp_host.cpp	61.11%	6 Missing and 1 partial ⚠️
tests/gtest/test_rcp_host_api.cpp	88.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2559      +/-   ##
==========================================
- Coverage   55.77%   46.31%   -9.47%     
==========================================
  Files          87      104      +17     
  Lines        6890    12484    +5594     
  Branches        0      906     +906     
==========================================
+ Hits         3843     5782    +1939     
- Misses       3047     6389    +3342     
- Partials        0      313     +313     

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[sunytt]

LGTM 👍