Add API support for Time Tracking

Description

In 8.14 we will ship Time Tracking as a Product. We won't have an API for that.

The sooner we have APIs for this Product, the faster we'll see integrations that take advantage of it (IDEs, other SAAS, or companies that build custom reports as they do now with all other features we have).

Proposal

• Add new API methods for time tracking:

  • Read: total time spent for an issue/MR
  • Write: individual time spent for an issue/MR (add up to previous entries)
  • Read: estimate for an issue/MR
  • Write: estimate for an issue/MR (overwrite previous entries)

• This API has to be restricted to EE Premium.

Out of scope:

• Read: list of individual time spent entries per issue/MR

/cc @smcgivern