[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:

trufflehog/pkg/sources/github_experimental/object_discovery.go

Lines 429 to 432 in 6d1c59f

	req, err := http.NewRequest("POST", "https://api.github.com/graphql", bytes.NewBuffer(requestBody))
	if err != nil {
	return nil, fmt.Errorf("failed to create request: %w", err)
	}

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

Checklist:

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