Support for TracerResolver.

[tomas-langer]

Fixes #1384
Signed-off-by: Tomas Langer tomas.langer@oracle.com

OpenTracing contrib includes a way to add tracer using a service loader. We have explicit support for Jaeger and Zipkin, this would add a generic support for any tracer, although without support for configuration using Helidon config - the configuration is ignored except for:

# Whether the tracer is enabled (defaults to true)
enabled: true
# Service name - used only to log a statement, IGNORED for the actual tracing
service: "name"
# Whether to register this tracer as a global tracer (defaults to false)
global: true