CI variables of type file

What does this MR do?

Add new column variable_type to ci_variables, ci_group_variables, ci_pipeline_variables, and ci_pipeline_variables and introduce env_var (default) and file as possible values.

For variables of type file set file: true when exposing them to the runner. This will make the runner to write the value of the variable to a temporary file and set the path to this file as value of the ENV var named after the key. This runner behaviour was implemented in gitlab-runner!413 (merged).

EE MR - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12280.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #46806 (closed)