Add basic JSON helpers

What does this MR do and why?

Describe in detail what your merge request does and why.

Adds some basic JSON helpers which provide a shorter form of using the Gitlab::Json methods which will soon take precedence over using .to_json in views.

See !101067 (merged) for additional context.

This MR doesn't add any specific uses of these yet as they'll be used in autocorrects from rubocop in a subsequent MR.

Related #217633 (closed)

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