Refactors all references to ExternalApprovalRules to
ExternalStatusChecks to more properly represent the
features purpose.

Changelog: changed

List status checks for a merge request

Adds a new API endpoint to list all approval rules (status checks)
for a merge request and their status.

List status checks for a merge request

Adds a new API endpoint to list all approval rules (status checks)
for a merge request and their status.