feat: expose SigstoreTUFClient mocks

[ramonpetgrave64]

Following #768

For folks using slsa-verifier as a library, it could be useful to export the mocks we already have for the TUF client newMockSigstoreTUFClient, and it's implementation for GetTarget. We may also include some sample data for the mock.

slsa-verifier/verifiers/internal/gha/npm_sigstore_tuf_test.go

Lines 88 to 106 in 97ea5f8

	// mockSigstoreTUFClient a mock implementation of sigstoreTUFClient.
	type mockSigstoreTUFClient struct {
	fileContentMap map[string]string
	}
	// newMockSigstoreTUFClient returns an instance of the mock client,
	// with fileContentMap as input and outputs of the GetTarget() method.
	func newMockSigstoreTUFClient() *mockSigstoreTUFClient {
	return &mockSigstoreTUFClient{fileContentMap: mockFileContentMap}
	}
	// GetTarget mock implementation of GetTarget for the mockSigstoreTUFClient.
	func (c mockSigstoreTUFClient) GetTarget(targetPath string) ([]byte, error) {
	content, exists := c.fileContentMap[targetPath]
	if !exists {
	return nil, fmt.Errorf("content not definied in this mock, key: %s", targetPath)
	}
	return []byte(content), nil
	}

[haydentherapper]

Can we upstream or align this with the existing testing in sigstore-go? In https://github.com/sigstore/sigstore-go/blob/main/pkg/tuf/client_test.go, we have stubbed out the TUF repo so that you can initialize a real client that doesn't make outbound network calls.