Administration: Allow filtering of bulk action fields

[aaronjorbin]

This updates the patch from @ethitter to include inline documentation and a single filter

https://core.trac.wordpress.org/ticket/63005

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[joedolson]

As a private variable used internally, I think that bulkFieldRelations is fine, but as an externally available filter, I think it's confusing; it doesn't really say anything about what this is used for.

What would you think about changing to something like bulkActionObserverIds ? This is really the array of field IDs that will be used to verify that a bulk action is taking place; the naming and documentation could make this a lot clearer.

[kabir-coderex]

Can we use optional chaining here for cleaner and safer code?

[kabir-coderex]

Can we use optional chaining here for cleaner and safer code?