Implement a GHA action to scan for feature flag references

[emyller]

In github.com/Flagsmith/flagsmith-ci.
Using the Docker image created on #5705.

We need a GitHub Actions action to scan for feature flag references in a codebase. It cannot be a reusable workflow, because it needs access to the code within the GHA runner. See documentation.

It needs to output the feature flag references JSON provided by #5705, which will be used by a reusable workflow (#5708) to push code references to Flagsmith.