✨ Add new custom-composer-filename option

[badasukerubin]

Description

As discussed in #258 .

This adds a new custom-composer-filename option to the action, allowing for adding a custom composer filename, which can be helpful for using composer based on different environments.

See: https://getcomposer.org/doc/03-cli.md#composer

Fixes #258.

Motivation and context

See: #258

How has this been tested?

Both unit tests as well as integration tests are included with the PR.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

PR checklist

• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING.md document.
• [x] I have added tests to cover my changes.