Better casting in ViewableData

[GuySartorelli]

Right now the $casting configuration on ViewableData only kicks in if the value that should be cast is not already an object. This means if a method returns some instance of DBField the casting configuration will be ignored.

The casting also doesn't care what the original value is - so the default cast of Text will cast e.g. booleans to DBText instead of to DBBoolean

More recently this caused problems when trying to allow push associative arrays to templates in #11237. We wanted to treat associative arrays as ArrayData, but this resulted in bypassing casting, which caused problems in various areas of kitchen sink.

Notes

• There's a fair amount of duplicated logic regarding this between ViewableData and SSViewer_Scope - if feasible, they should be deduped and share logic between them.
• Refer to this diff for when casting to ArrayData was previously removed from a PR - this may be useful e.g. for test coverage

Acceptance Criteria

• A new "magic" value for the default_cast and casting configuration properties is added
  • This casts things intelligently - associative arrays become ArrayData and primitives become the appropriate type (boolean becomes DBBoolean, int becomes DBInt, etc)
• The default value for default_cast is set to the new value
• Kitchen sink is updated as necessary so that tests pass - e.g. some $casting config may need to be updated so specific associative arrays are still cast as Text, retaining existing behaviour
• SSViewer_Scope::convertScalarToDBField() is updated to use the default_cast configuration value

[GuySartorelli]

Done as part of #11322