Support Converter for SpEL return types to AuthorizationDecision

[rwinch]

Support custom return types for for SpEL method security through the use of providing a Converter of that return type to the Authorization Decision.

I don't think that I like this idea but I'm documenting it. The reason I'm not sure if I like this idea as the SpEL expression can invoke the converter itself.

[jzheaux]

Closed by automation due to typo in commit message. Reopening.