Make download more robust to ensure we get max dataset coverage

[maryamariyan]

Previously we used to sacrifice download time in order to get as many issue/PR data as possible.

Later we refactored the download code, so that the queries become faster while sacrificing a cut on the number of issues/PRs that get downloaded. e.g. when doing bulk queries, if the query contains a faulted PR or issue to download then the whole query fails.
For example: dotnet/runtime#203 is one example of a faulty PR.

To accommodate this, let's provide an option: to have a choice to either get a faster bulk download or a larger coverage of all possible issues.

Currently the default is set to do a fast download.
To change that would need to change this flag:

issue-labeler/src/CreateMikLabelModel/DL/DownloadHelper.cs

Line 26 in ec8ed46

bool fastDownload = true;