[pd] deprecate old prefixes in radvd configuration on DHCPv6 PD change

[yangsong-cnyn]

This PR addresses an issue where old prefixes were not being properly deprecated in the RADVD configuration when the upstream interface's delegated prefix changed. Previously, only the new prefix would be advertised, potentially leading to issues for OT still using the old prefix.

Now, upon a new PD message, dhcpcd.enter-hook will:

• Identify the old and new prefixes: It retrieves the old and new prefix information from the dhcpcd environment variables.

• Configure radvd to deprecate the old prefix: If an old prefix exists and is different from the new prefix, the radvd.conf file is updated to include the old prefix with a preferred and valid lifetime of 0. This signals to OT that the prefix is deprecated and should no longer be used.

• Publish the new prefix: The new prefix is added to radvd.conf with the appropriate lifetimes received from the DHCPv6 server.

• Reload radvd: The radvd service is reloaded to apply the changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.29%. Comparing base (2b41187) to head (b6fb0f4).
Report is 941 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2701       +/-   ##
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- Coverage   55.77%   43.29%   -12.49%     
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  Files          87      108       +21     
  Lines        6890    13400     +6510     
  Branches        0      963      +963     
===========================================
+ Hits         3843     5801     +1958     
- Misses       3047     7291     +4244     
- Partials        0      308      +308     

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