Feature: Add support for GitLab releases

## Summary

Bring a parity of functionality to release on GitLab instances.

## Details

GitLab has parallel, if not identical functionality for pages and Release artifacts.

After looking through [pkg/github/github.go](https://github.com/helm/chart-releaser/blob/main/pkg/github/github.go), most of the necessary functionality is present within the `glab` CLI (links below) to provide the functionality as needed.

Supplementally, GitLab also has support for Helm objects within the [package repository](https://docs.gitlab.com/ee/user/packages/helm_repository/). That can be published to via API calls (example in `gitlab/gitlab` chart's CI [here](https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/.gitlab-ci.yml#L581)), but is not currently implemented by an existing library call within `glab`.

## Reference

- `glab` GitLab CLI: https://gitlab.com/gitlab-org/cli
- [glab release](https://gitlab.com/gitlab-org/cli/-/tree/main/docs/source/release) sub-command
- [release libraries](https://gitlab.com/gitlab-org/cli/-/tree/main/commands/release) within the project
- `pkg/github` [implementation](https://github.com/helm/chart-releaser/blob/main/pkg/github/github.go)