Support conversin of enums with JAXB's @XmlEnum

The string to enum converter relies on the enum valueOf method. This does not play well with enums created by xjc or annotated with @XmlEnum. This improvement should detect that annotation and use the fromValue method rather than the valueOf method.
#### Affected Versions
[2.7]

Locate Jersey's string-to-enum converter and inspect how it currently invokes valueOf. Check the JAXB @XmlEnum and generated fromValue behavior, then add coverage for annotated or xjc-generated enums and verify conversion uses fromValue while ordinary enums retain their existing behavior.

Written by the indexing model from the issue text.