[memory observer] Allow users to easily export existing data before reducing memory

Some DevTools screens support exporting the current data. For screens that support this, we can add a way for users to export data before reducing memory when they click "Reduce memory" in the memory pressure warning. Right now, the warning does tell users what the consequences of clicking this button will be and we recommend that they export any data they do not want to lose.

Follow up feature for https://github.com/flutter/devtools/pull/8998.