Push decision to use credentials or not down to where the creds are used

[robertbrignull]

This fixes a potential bug I spotted where we might unexpectedly prompt a user for credentials to download a CodeQL database.

The code for downloading databases from GitHub is in promptImportGithubDatabase and downloadGitHubDatabase and both of those took an optional Credentials parameter. It is then up to the caller to decide whether to use credentials or not. In this PR we push the decision further down so we only decide in downloadGitHubDatabase and therefore use the same logic for all cases.

The current logic around using credentials is:

• In handleChooseDatabaseGithub in local-databases-ui.ts (called when manually importing a database by command palette or UI), we were using credentials only when the canary flag was set.
• In createSkeletonQuery in local-queries.ts, we were using credentials only when the canary flag was set.
• In promptImportDatabase in model-editor-view.ts (called when modeling a dependency or generating modeled methods), we were always using credentials.

This seems like a bug to me as the rest of the model editor doesn't require canary mode, and in all other instances of downloading a database it doesn't prompt for credentials when not in canary mode.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[koesie10]

LGTM. I agree that we shouldn't be using the credentials in the model editor in non-canary mode, especially since this would only result in a credentials prompt when using flow generation.

[robertbrignull]

I realised that this is a user-visible change so it should probably have a changelog entry. I hope that's ok. If you disagree we can editor or remove it again.