Implement Java agent application context retrieval

Implement Java agent application context retrieval

Java application can now register an IContextInfoRetriever to provide
context information. This information can be used for filtering:

  lttng enable-event -j myevent --filter '$app.retriever:context=="something"'

or for saving in the trace directly by enabling the context:

  lttng add-context -j -t '$app.retriever:context'

See the "ApplicationContextExample.java" program for an example of
utilization.

Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>