[GitHub] Add a GraphQL client to the connector

[rgmz]

Description:

This introduces a latent GraphQL client without making any functional changes. The GraphQL API is required for certain features (e.g., #1906), and it is already being manually called in at least once place:

https://github.com/trufflesecurity/trufflehog/blob/6d1c59f018be0ea3b37aec7fc4b12815c91ab69a/pkg/sources/github_experimental/object_discovery.go#L429-L432

shurcooL/githubv4 is the library recommended by the authors of google/go-github.

Checklist:

• [x] Tests passing (make test-community)?
• [x] Lint passing (make lint this requires golangci-lint)?