Add option to make manual variables masked in `.gitlab-ci.yml`

Proposal

Add option to make manual variables masked. This feature will use the same mechanism that Project or Group variables use.

New keyword variables:masked is introduced with true or false value. Default value for this option would be false.

Example:

Variable definition in .gitlab-ci.yml:

variables:
  TOKEN:
    description: "Enter your token."
    masked: true
  URL:
    description: "Enter Auth URL."

This feature would be really useful in manual pipelines, where user needs to enter some input which company prefers to mask in job outputs. It will be an addition to the existing global keywords variables:description and variables:options specifically created for manual pipelines.

New dropdown menu in Pipeline Run UI is optional for this FR.