[border agent] add UDP Proxy module for Thread commissioning

[Irving-cl]

This PR adds a UDP Proxy module to do Thread commissioning under NCP architecture.

This PR is one of sub-task to support Border Agent under NCP mode. The UDP Proxy will also be used to do Thread commisssioning under RCP architecture and replace OT_PLATFORM_UDP.

Based on the OT border agent state changes (HandleBorderAgentStateChange), the module simply starts/stops listening on an UDP ephemeral port and if starts, sets a mapping between OT border agent UDP port and the port on host. And then it does bidrectional UDP forwarding:

• Dependencies::UdpForward will be implemented as OT UDP forwarding to OT core.
• SendToPeer will send a UDP packet to the remote peer using the local host port (that maps to OT border agent port) as the source port.

The module hasn't been integrated with the application and put in use. This PR adds a unit test for the module.

This module can also be used for other cases.

[codecov]

Codecov Report

Attention: Patch coverage is 74.57627% with 45 lines in your changes missing coverage. Please review.

Project coverage is 43.82%. Comparing base (2b41187) to head (8ddb9a9).
Report is 975 commits behind head on main.

Files with missing lines	Patch %	Lines
src/host/posix/udp_proxy.cpp	77.14%	20 Missing and 4 partials ⚠️
tests/gtest/test_udp_proxy.cpp	68.65%	16 Missing and 5 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2717       +/-   ##
===========================================
- Coverage   55.77%   43.82%   -11.95%     
===========================================
  Files          87      113       +26     
  Lines        6890    13569     +6679     
  Branches        0      965      +965     
===========================================
+ Hits         3843     5947     +2104     
- Misses       3047     7303     +4256     
- Partials        0      319      +319     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Irving-cl]

@bukepo @abtink

Could you help take a look at this PR when you have a chance? This one is a little bit long so maybe first let's align on the interfaces of the module UdpProxy.

For now I didn't add a general UDP platform module because the border agent is the only user. We can refactor it into a general UDP platform module when there are more users.

[bukepo]

Great to see we are on the way to deprecate platform UDP.

[Irving-cl]

@bukepo As discussed, updated the module to be generic so that it can also work for other cases (not only for Border Agent).

[abtink]

LGTM.