feat: add report_on flag to flutter package

[marcossevilla]

Description

Currently the flutter_package workflow doesn't have any report_on parameter like dart_package does:

report_on:
        required: false
        type: string
        default: "lib"

This is useful since due to Pub Workspaces we now have to use flutter_package in CI because it uses flutter pub get by default, so this makes it easier to set custom folders on Dart-only packages to report to coverage.

Requirements

• All CI/CD checks are passing.
• There is no drop in the test coverage percentage.

Additional Context

No response

[alestiago]

We think this is a valid request 🙌🏻

Unfortunately @marcossevilla doesn't have enough bandwidth for the time being to tackle this effort. I'll leave it unassigned for now until there is an interested contributor.