Add include_environments_lockfile configuration option

[danyeaw]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

This is part of conda-incubator/conda-self#54. This would be implemented after #1050.

Add a new boolean configuration key to constructor.yaml to control lockfile generation.

Why is this needed?

Users should be able to opt-in/out of including the lockfile, and it provides clear control over the feature.

What should happen?

We need to add parsing for the include_environments_lockfile key in the constructor configuration system. The default value should be false for backward compatibility. This work should include validation and documentation for the new key, adding it to the constructor.yaml documentation with examples, and unit tests for configuration parsing and validation.

Additional Context

No response

[danyeaw]

Closed in favor of the approach in #1052.