Time tracking API: list of individual time spent entries per issue/MR

Description

You can add many /spend entries in an issue/MR. We have the total amount with the API (https://gitlab.com/gitlab-org/gitlab-ee/issues/1228), but we don't have the history. Users might want to have the history of the time spent per issue/MR in order to have a breakdown of who did what, when.

Proposal

• Have a method to get all the /spend entries about an issue/MR
• This method should also indicate who entered the time
• Restricted to EE Premium