Add keyAndEntityFieldsDatabase function to return key with migration only fields

[josephsumabat]

This PR adds a version of the keyAndEntityFields function which includes MigrationOnly and SafeToRemove fields. We would like to make use of this in persistent-documentation in order to generate documentation for fields marked by those modifiers.

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[parsonsmatt]

Thanks, looks great! Do you mind adding a Changelog entry? I haven't released 2.14.6.0 yet so we can fold this in there.

[parsonsmatt]

This exported too, mind adding a doc comment and @since?