Added stale-if-error and fixed bug for frontend cache staleWhileRevaidate config (#21886)

[JamesMarino]

This PR fixes the staleWhileRevaidate config not being passed into the cacheControl() middleware if it is set via the frontend caching config. It also adds the staleIfError configuration to allow the stale-if-error Cache-Control header directive.

Tests have also been added for varying cases of the directives including when the Ghost config is not set.

Documentation from https://ghost.org/docs/config/#caching will also need to be updated. Happy to update this given the source.

• There's a clear use-case for this code change, explained above
• Commit message has a short title & references relevant issues
• The build will pass (run yarn test and yarn lint)

Closes issue #21886