[ncp] integrate netif multicast address subscription

[Irving-cl]

This PR integrates the netif multicast address subscirption update with the NCP.

With this PR, applications can listen to a multicast address on wpan interface and receive multicast messages from the Thread network.

The implementation of NcpSpinel::Ip6MulAddrUpdateSubscription uses SPINEL_CMD_PROP_VALUE_INSERT and REMOVE. This PR extends NcpSpinel to support using these commands.

It's a little hard to add an integration test for this. Because there isn't any native tools to easily listen on a multicast address and check what has received. I've verified it locally with a simple c++ program.

[codecov]

Codecov Report

Attention: Patch coverage is 3.17460% with 61 lines in your changes missing coverage. Please review.

Project coverage is 45.48%. Comparing base (2b41187) to head (6b4e893).
Report is 809 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ncp/ncp_spinel.cpp	0.00%	54 Missing ⚠️
src/ncp/posix/netif.cpp	22.22%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2498       +/-   ##
===========================================
- Coverage   55.77%   45.48%   -10.30%     
===========================================
  Files          87      100       +13     
  Lines        6890    11875     +4985     
  Branches        0      870      +870     
===========================================
+ Hits         3843     5401     +1558     
- Misses       3047     6181     +3134     
- Partials        0      293      +293     

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[sunytt]

LGTM, thanks!

[abtink]

LGTM. 👍