Allow usage of Dashboard Variable Filters that are not defined in Shared Dashboards

With OPT-3263 we allow users to define deeplinks into filtered dashboards using the Sharing functionality. 

There are use cases where users would like to manipulate the share link on their own to pass in values that are not yet defined in the shared dashboard. However, in the current implementation, we validate that the filters and values have been defined in the dashboard whenever the parameters are passed in via the URL. This is due to the fact that there are many use cases where the dashboard owner / editor does not want that additional filter parameters are available to the viewer of the share url and we secure this functionality.

There are two potential solutions:

• In order to support the new use case, we could think about adding another checkbox to the sharing panel which allows to define if users are allowed to pass in parameters or filters that are originally not defined. This would broaden the use cases for the sharing and potentially more users would use to Optimize and embed dashboards into their applications.
• Offer a free-text field for Variable Value Filters (can be activated when adding a variable filter for a variable). Once such filter has been added to a Dashboard and users activate "Share dashboard with active filters" then users can pass in any variable value for this variable filter in the URL. 

Additional Context:

We have received feedback from at least two users who do not want to specify all available values for variable filters upfront, but rather would allow the viewer a lot of flexibility. This is especially useful if viewers want to filter for specific "cases", let's say a specific order, identified by an orderID. It is also useful if when setting up the dashboard and its filters, not all values are known upfront.