[dartgen] Add ability to customize format options for dartgen command

[fondoger]

Currently, when running goctl api dart ... to generate dart API files, the dart formatter will be triggered automatically. But it always uses the default configurations when formatting the dart files.

This PR adds a feature to allow users to override the default dart format parameters. So the formatted dart file obeys the lint rules of the existing dart project.

Before:

goctl api dart -api server.api -dir ../app/api

=> `dart format <dir>`

After:

goctl api dart -api server.api -dir ../app/api -format-args "--line-length=150"

=> `dart format <dir> --line-length=150`

The current behavior is not affected.

[codecov]

Codecov Report

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Additional details and impacted files

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