fix: fix issue #1325 and add missing `truncated` field in the `GHGist` object

[SCHJonathan]

Description

1. Fixes #1325 by adding a lazy download from the raw_url field of GHGistFile object if the gist file content is uninitialized.
2. From the list public gist documentation, I found out we are missing the truncated field in the GHGist object and I added that field in this PR.

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[SCHJonathan]

@bitwiseman. Thank you so much for your help and detailed comment, bitwiseman! All of those suggestions are super helpful and I have made revisions based on your suggestions!

[bitwiseman]

@SCHJonathan You're very close on this. Do you time to finish this PR in the next few days? I plan to do a release soon.