API Endpoint for SAML Group Sync
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### Proposal
Create new endpoints for SAML group sync.
SAML Group Sync was introduced in #118 but only via the UI. This is to propose to make this accessible via the API.
These should be new `list/add/delete` endpoints (ex: `/groups/:id/saml_group_links`)
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### Availability & Testing
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What risks does this change pose to our availability? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing?
Please list the test areas (unit, integration and end-to-end) that needs to be added or updated to ensure that this feature will work as intended. Please use the list below as guidance.
* Unit test changes
* Integration test changes
* End-to-end test change
See the test engineering planning process and reach out to your counterpart Software Engineer in Test for assistance: https://about.gitlab.com/handbook/engineering/quality/test-engineering/#test-planning -->
**What risks does this change pose to our availability?**
This feature is low risk for GitLab.com and self-managed availibility.
**What additional test coverage or changes to tests will be needed?**
Add API tests for the new end points.
**Will it require cross-browser testing?**
N/A
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