Subscriptions accept labels that don’t exist in the project

When adding a GitLab subscription with a label feature, the plugin does not check if the label actually exists in the project. The subscription is still created even when the label is invalid.

Steps to Reproduce:-

Set up GitLab with Mattermost.
Run /gitlab subscriptions add owner[/repo] issues,merges,label:"label1" (where label1 does not exist in the project)
The subscription is created without error.

Expected behavior
The plugin should validate that the label exists in the project. If the label does not exist, the subscription should not be created, and an error message should be returned.

Observed behavior
The subscription is created successfully even though the label does not exist in the project.

Start by tracing the /gitlab subscriptions add command and the handling of the label feature. Reproduce the case with a nonexistent label, then verify that invalid labels prevent subscription creation and return an error.