[nexus] implement test 1.2.LP.7.2.1 for Forward Tracking Series (#12624)

This commit implements Nexus test 1.2.LP.7.2.1 to validate the Forward
Tracking Series Link Metrics functionality.

Specifically, the test verifies that:
- The DUT (Leader) correctly handles Forward Series Link Metrics
  Management Requests from SED and SSED children.
- The DUT properly aggregates metrics for Forward Series (MAC Data
  Requests for SED, all data frames for SSED).
- Aggregated results are accurately reported in MLE Data Responses
  when queried.
- Forward Series can be successfully cleared, and unknown Series IDs
  result in appropriate error statuses.

Summary of changes:
- Created test_1_2_LP_7_2_1.cpp to implement the test logic.
- Created verify_1_2_LP_7_2_1.py for automated packet verification.
- Updated verify_utils.py to include MLE TLV field definitions for
  Link Metrics (e.g., forward series, flags, query ID).
- Registered the new test in tests/nexus/CMakeLists.txt and
  tests/nexus/run_nexus_tests.sh.