Respect MergeQueue bool flag when querying github PRs

[pleicht]

Disclaimer: I have never written Go code before.

For enterprise clients on a github version < 3.11 MergeQueue support isn't implemented. As a result

mergeQueueEntry {
    id
}

isn't valid inside of the pull request query.

This is logically the correct way to do this, but I understand two of these files are generated and I shouldn't edit them, however I'm unsure of how to actually generate client.go and operations.go.

If someone can point me in the right direction I can update the PR to generate client.go and operations.go correctly

Closes #356

[ejoffe]

@pleicht : Thank you for your pr, I think I found a way to acheive this without the hacks inside the generated code.
Please checkout PR #370. If you can test it and validate that it works, I'll release the fix.