feat: add environment variable interpolation to YAML script parser

[georgezlei]

Add YAML Configuration Parser with Environment Variable Support

Environment variable interpolation is very useful when sensitive data, such as user passwords and other PII data is used in the test. This PR introduced a {{VAR_NAME}} syntax in the YAML script to interpolate the environment variables.

Changes

• Implemented environment variable interpolation in YAML configuration parser.
• Added unit tests for YAML utilities including building, parsing, and flow item brief generation

Features

• Environment variable interpolation using {{VAR_NAME}} syntax
• Throws descriptive errors for undefined environment variables

Testing

Added test suite covering:

• Environment variable interpolation
• Error handling for undefined variables

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[yuyutaotao]

Hi @georgezlei

Great job !

How about we replace the {{var_name}} syntax to the ${var_name} syntax? The ${var_name} syntax looks tidier and similar to the JavaScript template string syntax.

And remember to sign the CLA in this PR .