Implement Generic() func on Client

[gprossliner]

This PR allows to call any API that is not (yet) implemented by the module itself, like "/authz/shares":

func shareStream(context context.Context, streamID string, userID string)  error
{
	request := struct {
		Selected_grantee_capabilities map[string]interface{} `json:"selected_grantee_capabilities"`
	}{map[string]interface{}{
		"grn::::user:" + userID: "view",
	}}

	grn := "grn::::stream:" + streamID
	_, err = client.Generic(ctx, http.MethodPost, "/authz/shares/entities/"+grn, request, nil)
	return err
}

To make my fork build successfully, I had to execute go mod tidy.
I did not commit the resulting changes to go.sum to avoid merge-conflicts.

I tried to keep the coding-style like the existing files.

I added unit-tests for the constructing the Endpoint URL, but no integration tests incl. Mock-Server.
Please let me know if you are interested in merging this PR, than I can implement these as well.

[m-barthelemy]

Would this PR help for #332 ?
Will it get merged?

[gprossliner]

Hi! This would not implement #332 directly, but allow the user to call it, or any other / furture API methods not directly implemented in go-graylog.

Because I got no answer on this PR or #322 issue, I did not proceed further. To avoid implementing this totally twice (one by the go-graylog client, and once by just http.Client, I copied the Source files into my project, and use the "Generic" Implementation from this PR.

Nevertheless I would like to get this merged, or at least some feedback.