Specify optional dependency on sun.misc.Unsafe

[Marcono1234]

Resolves #1695 (though maybe Gson documentation could be improved to mention usage of Unsafe)
(Hopefully) resolves #1981

I have marked this as draft because I am not very familiar with OSGi. To be tested:

• works for OGSi deployments without Unsafe
• works for OSGi deployments with Unsafe and Gson is able to construct instances of class without default constructor (by using Unsafe)
  • With System property org.osgi.framework.bootdelegation=sun.misc.*
  • With System property org.osgi.framework.system.packages.extra=sun.misc
  • Using extension bundle from https://github.com/diffplug/osgiX (?)

See also this StackOverflow answer and diffplug/osgiX README.

Ideally a test as proposed by #1993 should be added as well (with some adjustments), but the class from #1993 cannot be reused because the CLA is not signed.

[JaroslavTulach]

Ideally a test as proposed by #1993 should be added as well (with some adjustments)

I have signed the CLA meanwhile. Thanks for your fix. I merged your commit 91a780a into #1993 and adjusted the test.