GitLab CI option in template

[nloum]

What would you like to be added:

In addition the Azure Pipelines and GitHub Actions options in the unoapp dotnet new template, it would be awesome if we had an option for GitLab CI.

Why is this needed:

There are three main git hosting platforms that have their own CI: Azure DevOps (Azure Pipelines), GitHub (GitHub Actions), and GitLab (GitLab CI). It makes sense to support these out of the box. GitLab has an estimated ~30 million users according to Wikipedia.

For which Platform:

• [ ] iOS
• [ ] Android
• [ ] WebAssembly
• [ ] WebAssembly renders for Xamarin.Forms
• [ ] Windows
• [x] Build tasks

Anything else we need to know?

I'd be willing to contribute it if this is a direction the team wants to go. I know that there are other feature requests about adding a lot more into CI (#679), so there will be a maintenance burden to having to support three CI providers.