Add lockbox gem to support encrypting files

What does this MR do?

As part of #207401 (closed), add the lockbox gem to help with encryption/decryption of terraform state files.

A POC of how it will be used can be found in !26116 (closed).

Some details about lockbox:

• does not depend on any other gems
• uses the AES-GCM algorithm by default
• uses OpenSSL to perform cryptography
• comes with CarrierWave support (encrypt files by adding a single line to an Uploader)
• uses a different key for each database field and file uploader (derived from the master key)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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