Add support for client-defined connection protocols

[andrewazores]

This adds a clientlib directory and handling for it, which allows end users to supply additional JARs to be added to the Cryostat classpath. This allows for various extensions to Cryostat, ex. it would allow for a reflection-based platform client or auth manager selection (again), but in this instance it is intended to allow Cryostat to communicate with targets using alternate service types than jmxrmi, such as Wildfly's remote+http.

The new Wildfly sample application in smoketest.sh listens using remote+http on port 9990, so with this jboss-client.jar in /clientlib, Cryostat can connect to the Wildfly server using the URL service:jmx:remote+http://localhost:9990. This target will need to be manually added using the Custom Targets HTTP API (#472). Example using HTTPie:

$ http --verify=false -f POST https://0.0.0.0:8181/api/v2/targets connectUrl="service:jmx:remote+http://localhost:9990" alias=Wildfly
HTTP/1.1 200 OK
content-encoding: gzip
content-length: 144

{"meta":{"type":"application/json","status":"OK"},"data":{"result":{"connectUrl":"service:jmx:remote+http://localhost:9990","alias":"Wildfly"}}}

See the below attachment for such a jboss-client library that can be used - just rename .zip to .jar (GitHub does not allow .jar uploads but does allow .zip...)

jboss-client.zip

Fixes #469

Question/TODO: support uploading client libraries through an API handler, similar to client certificates?

[ebaron]

Seems to work as expected