Add last_used_at field to cluster agent token

What does this MR do?

We want to be able to display to a user the last time a cluster agent token was used.

Related issue: #322128 (closed)

Related design https://gitlab.com/gitlab-org/gitlab/uploads/0fecb603cd918fdeb6e9f2d6178d03fe/000__Agent_details_page-Access_tokens_tab.png

This MR is the first part of the issue, which will add the field last_used_at to the database table.

Migration

up

== 20210309181019 AddLastUsedAtToClusterAgentToken: migrating =================
-- add_column(:cluster_agent_tokens, :last_used_at, :datetime_with_timezone)
   -> 0.0055s
== 20210309181019 AddLastUsedAtToClusterAgentToken: migrated (0.0056s) ========

down

== 20210309181019 AddLastUsedAtToClusterAgentToken: reverting =================
-- remove_column(:cluster_agent_tokens, :last_used_at, :datetime_with_timezone)
   -> 0.0055s
== 20210309181019 AddLastUsedAtToClusterAgentToken: reverted (0.0112s) ========

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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• 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

Related to #322128 (closed)