feat: add support for GitLab CI environment

[maxrake]

The phylum Python package has been updated to offer the ability to analyze lockfile changes within the GitLab CI/CD environment. Merge requests (MRs) can be analyzed and annotated automatically when they include changes to a lockfile.

The core logic and annotation strategy came from existing Phylum implementations:

• The phylum-dev/phylum-analyze-pr-action repository is the current working implementation for GitHub
• The phylum-dev/GitLabCI-Phylum-Analyze-MR repository is the current implementation for GitLab
• The intent was to replicate as much as what was already available...but in an extensible way
  • There were a few deviations and licenses taken where it seemed prudent
• Make use of the package's ability to download and install the latest released Phylum CLI
• No other external GitLab-based dependencies were used
  • Functionality was added to the package itself to do what was needed in the form of direct system/app and API calls

The best bet for learning about the functionality added here is to start with the README.md and docs/gitlab_ci.md documentation files. The code in this PR has been exposed as a Docker image and pushed to the phylumio/phylum-ci:gitlab tag. That image can be used to test locally or with your own GitLab private repository. In fact, it can be used now, for any customer that is hot to get started (CC: @furi0us333)

Closes #31

Checklist

• Does this PR have an associated issue (i.e., closes #<issueNum> in description above)?
• Have you ensured that you have met the expected acceptance criteria?
• Have you created sufficient tests?
  • Not yet...and won't be in this PR
  • Extensive manual local testing was performed with a personal GitLab private repository
  • Unit and functional tests were skipped for now in the name of expediency (I know, I know...)
  • Integration level tests will be tackled in a separate issue and will make use of the phylum-dev/TestGitLabWorkflow repo
• Have you updated all affected documentation?
• Remove the temporary gitlab tag from Docker Hub after this PR is merged and a new release cut

[maxrake]

Some screenshots from a sampling of the manual testing performed:

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