CI/CD for third party hosted projects
### Description
Create CI and CD jobs and pipelines for projects hosted outside the GitLab instance.
### Proposal
Add a feature that lets users define CI/CD jobs and pipelines that run in response to signals from third party platforms. Like a webhook call from GitHub.
### Links / references
### Documentation blurb
#### Overview
##### What is it?
See description?
##### Why should someone use this feature?
To take advantage of GitLab-CI's feature set while still meeting the project needs that mean the project can't be hosted on GitLab itself.
##### What is the underlying (business) problem?
At work, our internal projects are hosted on an internal instance of Gitlab-CE. But we also have multiple open sourced projects hosted on GitHub. CI/CD for projects hosted on GitHub requires use of a non-GitLab CI/CD tool.
##### How do you use this feature?
To run CI/CD tasks for projects hosted on third party platforms like GitHub.
#### Use cases
A project is hosted on GitHub to enable collaboration outside a business, but the business still deploys the project to internal servers.
A company uses a non-git source control system but needs a solid CI/CD platform.
### Feature checklist
Make sure these are completed before closing the issue,
with a link to the relevant commit.
- [ ] [Feature assurance](https://about.gitlab.com/handbook/product/#feature-assurance)
- [ ] Documentation
- [ ] Added to [features.yml](https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/features.yml)
issue