Include User-Agent to all API requests
### Problem to solve
We don't know how many of our users are using the extension and we managed to push some extension changes in the past that started hitting GitLab.com API hard (https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/issues/211)
### Proposal
Include [User-Agent header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent) to all API requests made by the extension. This way we'll be able to see how many requests are coming from the extension.
```User-Agent: vs-code-gitlab-workflow/${packageJson.version}```
### Further details
<!-- Include examples, use cases, benefits, goals, or any other details that will help us understand the problem better. -->
### Links / references
For GitLab team members, the access logs can be seen in https://log.gprd.gitlab.net/app/kibana
issue