Adding access token prompt for GitLab Secure Files Match

[darbyfrey]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I see several green ci/circleci builds in the "All checks have passed" section of my PR (connect CircleCI to GitHub if not)
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

The authentication part of the GitLab Match storage backend can be confusing and the error messages are hard to understand. As reported in: #20525

Description

This change adds better error messaging as well as a prompt to supply the access token when it can't be found.

Testing Steps

1. To render the prompt, run fastlane match with gitlab_secure_files configured as the storage backend.
2. To render the error, run PRIVATE_TOKEN=foo fastlane match with gitlab_secure_files configured as the storage backend.

[darbyfrey]

@getaaron I've got another one for you when you have some time, thanks!