Feature Proposal: Refactor flutter-actions-suite into reusable bash functions for GitHub Actions & GitLab CI/CD

[Spaccesi]

Summary

Refactor the existing flutter-actions-suite steps into standalone bash functions so the same logic can be reused across both GitHub Actions workflows, GitLab CI/CD pipelines or other platforms — without duplicating code.

Motivation

Currently, the suite is tightly coupled to GitHub Actions syntax (e.g. uses:, with:, step contexts). This makes it impossible to reuse the same Flutter CI logic in GitLab pipelines or any other CI system without manually porting every step.

Proposed Approach

• Extract each action's logic into a self-contained bash function inside a shared shell script (e.g. flutter_ci_functions.sh)
• Each function should rely only on environment variables for configuration (e.g. FLUTTER_VERSION, BUILD_TARGET, ARTIFACT_PATH) so it is CI-system-agnostic
• GitHub Actions wrappers continue to exist but simply source the script and call the appropriate function
• GitLab CI jobs can source the same script in before_script / script blocks

Acceptance Criteria

• All existing GitHub Actions functionality is preserved
• Each logical step is a named bash function with documented env var inputs
• A GitLab CI template (.yml) is provided that sources and calls the functions
• README documents how to use the functions standalone (local dev / other CI)
• Existing GitHub Actions workflow tests still pass

Notes

• Functions should be POSIX-compatible where possible, or clearly document bash ≥ 4 requirements
• Secrets/tokens should always come from env vars, never hardcoded arguments