Allow tracker publisher to label publication targets

Current situation

The GitLab plugin in tools was updated by tools#92 (closed) and now allows to label publication targets with {prefix}::{quality gate result} labels using the respective opentf-ctl get qualitygate command option.

Tracker publisher uses this plugin to publish the quality gate results. Thus, it could be interesting (and relatively simple) to add to it the same functionality.

Desired outcome

Tracker publisher must handle gitlab/label: prefix labels in the workflow metadata. If such a label is present, the publication target will be labeled with the quality gate execution results ({prefix}::{quality gate result} label).