feat: rename --project to --gcp-project-id and add usage guidance

[devin-ai-integration]

This PR targets the following PR:

• feat: add secrets fetch command to CDK CLI #494

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Renamed the "--project" CLI argument to "--gcp-project-id" for improved clarity and added comprehensive usage guidance to the file docstring in _secrets.py. The docstring is also passed as help text for the secrets command group.

These changes make the CLI interface more intuitive and provide better documentation for users.

Link to Devin run: https://app.devin.ai/sessions/121e177095fa41d4ae43dcfae597f3de
Requested by: Aaron ("AJ") Steers

[devin-ai-integration]

Original prompt from Aaron:

@Devin - Take a look at the pyairbyte script called `examples/run_gsm_connector_secret_fetch.py`. I want to build the same into a CDK CLI, building on the work from this PR: <https://github.com/airbytehq/airbyte-python-cdk/pull/493>

[devin-ai-integration]

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