Add Deserializers.hasDeserializerFor() (and something for DeserializerFactory) to allow detection of explicitly supported types

[cowtowncoder]

Addition of "Safe Default Typing" in 2.10 is good for security, but can add complexity to actual allow-listing of types considered safe for purposes of polymorphic deserialization.

One thing that could significantly simplify adding rules would be ability to figure out types supported with explicit deserializers, provided by modules (esp. datatype ones). This should indicate all explicitly handled cases, but specifically NOT introspection based "POJO" (and similar for Scala, Kotlin) values.
If support method was added in Deserializers (and something for DeserializerFactory), we could add a mechanism in BasicPolymorphicTypeValidator to indicate something like "if type is explicitly supported, allow. This would then support things like:

1. Basic JDK String-like types (java.net.URL, java.util.UUID)
2. Basic (JDK) date/time types (java.util.Date / Calendar)
3. Strings, Booleans, number wrappers
4. JsonNode types.

and 3rd party types of Joda, Guava, etc.

[cowtowncoder]

Alas... while I was able to add supporting functionality wrt registered deserializers in 2.11, there is a big design flaw in PolymorphicTypeValidator which prevents implements this in 2.x without backwards incompatible change: PTV actually needs DeserializationContext as DeserializationConfig is NOT enough. Too bad it is not being passed even though it would actually be available.