More fined-grained control over triggering Bean Validation

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Currently, Bean Validation constraints on resources will always be validated as part of the JAX-RS request lifecycle. Allowing to turn off validation - e.g. for specific kinds of testing - seems desirable.

Similar to the BV integration of JSF and JPA, there could be a
JAX-RS-defined property for controlling the validation behaviour. It could look very similar to the property JPA defines for that purpose:

```
javax.persistence.validation.mode={auto|callback|none}
```

"auto" will perform validation during the entity lifecycle if a BV provider is present. So will "callback", with the difference that it raises an exception if no BV provider is present. "none" will disable validation.

Start by reviewing the JAX-RS request lifecycle and its Bean Validation integration; the issue names no files or tests. Define the property and its auto, callback, and none behavior, including how the absence of a Bean Validation provider is handled.

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