Add property GitLabCIBuildInfo.Source (#4431)

[ap0llo]

Add property Sourceto GitLabCIBuildInfo that exposes the value of the CI_PIPELINE_SOURCE variable that GitLab CI uses to indicate how a pipeline was triggered.

Since the variable has a finite amount of possible values, the property is modelled as an enum with constants for all values listed in the GitLab documentation (https://docs.gitlab.com/ee/ci/jobs/job_rules.html#ci_pipeline_source-predefined-variable).
To account for GitLab adding additional values in the future, GitLabCIBuildInfo.Source is a nullable enum that will return null in case the environment variable value cannot be converted to the enum.

This fixes #4431

[devlead]

LGTM 👍

[devlead]

@ap0llo your changes have been merged, thanks for your contribution 👍