Use a config value to choose whether to use the github API

[adityasaky]

Description

This is a follow up to a previous PR in gittuf/gittuf where we allowed configuring whether to use the API response or not for merge trees. In the app, this makes it configurable.

AI Usage

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  request.
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  pull request. I have described my use of AI below.

Contributor Checklist

• I have manually reviewed all content submitted to gittuf in this pull
  request.
• I fully understand the content I am submitting.
• The changes introduced are documented and have tests included if
  applicable.
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  Signoff.
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  Conduct.