[udp-proxy] track Mesh-Local prefix in UDP Proxy client

[wgtdkp]

Previous implementation requires the caller of the Commissioner library to set Mesh-Local prefix before
initiating MGMT commands. it is error-prone and breaks backward compatibility: we need to first fetch
the Active Operational Dataset and set the Mesh-Local Prefix to the commissioner library before sending
other MGMT_* commands:

ot-commissioner/src/app/commissioner_app.cpp

Lines 151 to 155 in 1b5dd47

	BbrDataset bbrDataset;
	SuccessOrExit(error = mCommissioner->GetActiveDataset(activeDataset, 0xFFFF));
	SuccessOrExit(error = mCommissioner->SetMeshLocalPrefix(activeDataset.mMeshLocalPrefix));
	SuccessOrExit(error = mCommissioner->GetPendingDataset(pendingDataset, 0xFFFF));

This PR refactors the UDP Proxy to have it manage the Mesh-Local prefix inside the ProxyClient class.

1. The Mesh-Local prefix is lazily requested from the Border Agent before sending the first MGMT command (if no Mesh-Local prefix is cached).
2. The cached Mesh-Local prefix is cleared when the commissioner receives MGMT_DATASET_CHANGED.ntf so that the latest Mesh-Local prefix will be fetched before next MGMT command.
3. The cached Mesh-Local prefix is cleared when the commissioner is disconnected.

In this way, the user doesn't need to be aware of the Mesh-Local prefix and no changes are required.

[codecov-io]

Codecov Report

Merging #188 (c8efa67) into main (f41edbd) will increase coverage by 0.00%.
The diff coverage is 93.33%.

❗ Current head c8efa67 differs from pull request most recent head de34d98. Consider uploading reports for the commit de34d98 to get more accurate results

@@           Coverage Diff           @@
##             main     #188   +/-   ##
=======================================
  Coverage   70.35%   70.36%           
=======================================
  Files          52       52           
  Lines        4780     4778    -2     
=======================================
- Hits         3363     3362    -1     
+ Misses       1417     1416    -1     

Impacted Files	Coverage Δ
include/commissioner/commissioner.hpp	43.75% <ø> (ø)
src/app/commissioner_app.cpp	39.65% <ø> (-0.10%)	⬇️
src/library/commissioner_impl.hpp	96.29% <ø> (ø)
src/library/commissioner_safe.cpp	79.92% <ø> (-0.31%)	⬇️
src/library/commissioner_safe.hpp	100.00% <ø> (ø)
src/library/commissioner_impl.cpp	75.91% <71.42%> (-0.66%)	⬇️
src/library/udp_proxy.cpp	91.13% <97.29%> (+5.42%)	⬆️
src/library/udp_proxy.hpp	91.66% <100.00%> (-1.20%)	⬇️

[codecov-commenter]

Codecov Report

Merging #188 (257d980) into main (1b5dd47) will increase coverage by 0.97%.
The diff coverage is 93.87%.

❗ Current head 257d980 differs from pull request most recent head d06e7c3. Consider uploading reports for the commit d06e7c3 to get more accurate results

@@            Coverage Diff             @@
##             main     #188      +/-   ##
==========================================
+ Coverage   69.40%   70.38%   +0.97%     
==========================================
  Files          52       52              
  Lines        4874     4782      -92     
==========================================
- Hits         3383     3366      -17     
+ Misses       1491     1416      -75     

Impacted Files	Coverage Δ
include/commissioner/commissioner.hpp	43.75% <ø> (ø)
src/app/commissioner_app.cpp	39.65% <ø> (-0.10%)	⬇️
src/library/commissioner_impl.hpp	96.29% <ø> (ø)
src/library/commissioner_safe.cpp	79.92% <ø> (-0.31%)	⬇️
src/library/commissioner_safe.hpp	100.00% <ø> (ø)
src/library/commissioner_impl.cpp	75.91% <71.42%> (-1.09%)	⬇️
src/library/udp_proxy.cpp	91.56% <97.56%> (+5.85%)	⬆️
src/library/udp_proxy.hpp	91.66% <100.00%> (-1.20%)	⬇️
src/library/coap.hpp	95.29% <0.00%> (-1.18%)	⬇️
... and 4 more