Add UUID for ci_runners entities

What does this MR do and why?

Add UUID for ci_runners entities

As described in Generate UUID for the ci_runners entry when new... (#594605 - closed), we're adding a globally unique UUID that's created at Ci::Runner entity creation.

To avoid heavy background migration adding missing UUID to existing entities across GitLab instances, a method to lazy-generate UUID at value read is added as well.

Closes #594605 (closed)

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