Provide a simple interface for trying to get a connector

At present, it is the responsible of the jsch-agent-proxy user to know which connectors should be attempted and in which order. While there are legitimate reasons for a user to want fine-grained control over this (attempting to minimize dependencies; only targeting a subset of the platforms jsch-agent-proxy supports; etc), there is currently no common-case way for a user to simply "make it work" without boilerplate (which may need to change with future library releases should an existing connector be deprecated, a new one being added, etc).

This patch introduces jsch.agentproxy.simple, which provides a single entry point for the connectors which are currently suggested for use. If it does not currently reflect best-practice selection, or if best-practices change in the future, it provides a single place where this can be updated, rather than needing to explicitly modify clients which may not necessarily need the more fine-grained control possible by directly depending on and using the individual connector subprojects.