examples/graphql_issues.rs: Demonstrate variables and paging

[Enselic]

This was quite tricky to figure out how to do (for me, anyway), so let's help others by demonstrating how to do it.

Example output:

$ GITHUB_TOKEN=github_pat_123ABC cargo run --example=graphql_issues
Page 1:
https://github.com/XAMPPRocky/octocrab/issues/435 Installation tokens are hard to re-use
https://github.com/XAMPPRocky/octocrab/issues/456 `IssueHandle::delete_label` panics
https://github.com/XAMPPRocky/octocrab/issues/463 [Bug] octocrab::Octocrab::request_installation_auth_token fails with status code 411
https://github.com/XAMPPRocky/octocrab/issues/474 Add Method For Viewing Repository Traffic Stats
https://github.com/XAMPPRocky/octocrab/issues/475 Add API Version configuration to the builder
Page 2:
https://github.com/XAMPPRocky/octocrab/issues/398 Improve Commits API
https://github.com/XAMPPRocky/octocrab/issues/417 Add Projects API
https://github.com/XAMPPRocky/octocrab/issues/421 How to access commit compare endpoint?
https://github.com/XAMPPRocky/octocrab/issues/422 Add `Status`/`Conclusion` enums directly to fields
https://github.com/XAMPPRocky/octocrab/issues/423 Add a `Page::all` method
Page 3:
https://github.com/XAMPPRocky/octocrab/issues/377 Generating Github REST API from Open-API
https://github.com/XAMPPRocky/octocrab/issues/380 Source closed when authenticating with personal token
https://github.com/XAMPPRocky/octocrab/issues/381 The Workflow Run Event Action `in_progress` is Not Supported
https://github.com/XAMPPRocky/octocrab/issues/394 Add method for downloading job logs
https://github.com/XAMPPRocky/octocrab/issues/396 Initializing octocrab takes 300ms on macOS

[XAMPPRocky]

Thank you for your PR!

LGTM, though I think as your graphql gets complex I would recommend using a graphql library with it. That might be another good example.