feat: support --flutter-version=fvm and --flutter-version=system

[eseidel]

I chose this approach so we didn't have to write any code to read .fvmrc files, and all we do is just know to run fvm flutter rather than flutter when determining the version from the system-available flutter.

Fixes https://github.com/shorebirdtech/shorebird/issues/1385

And is part of potentially fixing https://github.com/shorebirdtech/shorebird/issues/2529.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests.

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