Add examples/graphql_issues.rs

[Enselic]

I could not find any examples of a real graphql query, so here comes one. It would have helped me so hopefully it will also help others.

Example output:

$ GITHUB_TOKEN=github_pat_1234ABC cargo run --example=graphql_issues
Object {
    "data": Object {
        "repository": Object {
            "issues": Object {
                "nodes": Array [
                    Object {
                        "title": String("Add Method For Viewing Repository Traffic Stats  "),
                        "url": String("https://github.com/XAMPPRocky/octocrab/issues/474"),
                    },
                    Object {
                        "title": String("Add API Version configuration to the builder"),
                        "url": String("https://github.com/XAMPPRocky/octocrab/issues/475"),
                    },
                ],
            },
        },
    },
}
$ GITHUB_TOKEN=invalid cargo run --example=graphql_issues
GitHub {
    source: GitHubError {
        documentation_url: Some(
            "https://docs.github.com/graphql",
        ),
        errors: None,
        message: "Bad credentials",
    },
    backtrace: Backtrace(
           0:     0x5641fcb6af8a - backtrace::backtrace::libunwind::trace::h5d71e1a92685bd90
                                       at /home/martin/.cargo/registry/src/index.crates.io-6f17d22bba15001f/backtrace-0.3.68/src/backtrace/libunwind.rs:93:5
                                   backtrace::backtrace::trace_unsynchronized::h18e9f171dc178e01
                                       at /home/martin/.cargo/registry/src/index.crates.io-6f17d22bba15001f/backtrace-0.3.68/src/backtrace/mod.rs:66:5
           ...
        ,
    ),

[XAMPPRocky]

Thank you for your PR, and congrats on your first contribution! 🎉