API for configuring repository push mirroring via HTTP

Problem to solve

We support HTTP push mirroring via the UI but not the API only.

Further details

Support for HTTP pull mirroring was implemented in https://gitlab.com/gitlab-org/gitlab-ee/issues/1499

Proposal

Multiple pull mirroring targets are supported, so we probably need a new resource, like api/v4/projects/{id}/push_mirror

Links / references

Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/103762 (Internal use)