Allow calling generic APIs

[gprossliner]

There is a "callAPI" method:

go-graylog/graylog/client/util.go

Lines 32 to 35 in 3e39cb0

	func (client *Client) callAPI(
	ctx context.Context, method, endpoint string, input, output interface{},
	) (*ErrorInfo, error) {
	// prepare request

This is currently private. It would be nice if the User can call any APIs that are not implemented (yet), like /authz/shares. See #332.

I would suggest renaming callAPI to CallAPI to make it public, or provide a equivalent public method.

[gprossliner]

On implementing the PR, I figured out that just make the func public will not be as useful, as this doen't allow to pass in a relative URL. So I reimplemented it with a "Generic" endpoint. The existing "util.go" file remains unchanged.
New PR #335