[CICD] Support megatron-fl-lm integration test (#1142)

### PR Category
CICD

### PR Types
New Features 

### PR Description
## Summary

- Add a new GitHub Actions workflow
`functional_tests_megatron_fl_trigger.yml` that allows the
Megatron-LM-FL repository to remotely trigger FlagScale functional tests
via `workflow_dispatch`
- Cover both **standard training** and **heterogeneous training**
functional test scenarios

## Changes

New file: `.github/workflows/functional_tests_megatron_fl_trigger.yml`
(+476 lines)

### Workflow Design

1. **Trigger**: `workflow_dispatch` with three inputs:
- `megatron_lm_fl_repo`: Megatron-LM-FL repository path (e.g.,
`owner/Megatron-LM-FL`)
   - `megatron_lm_fl_branch`: Branch to clone and install
   - `platform`: Platform name (default: `cuda`)

2. **Jobs** (3 total):
- `checkout_and_config`: Checks out FlagScale, loads platform config via
`yq` YAML parsing, and outputs test matrices and environment settings
- `functional_test_train`: Runs standard training functional tests in
parallel based on the test matrix
- `functional_test_hetero_train`: Runs heterogeneous training functional
tests in parallel based on the hetero test matrix

Author: Darryl233