[sshd agent] Introduce ConnectorDescriptor

[sshd agent] Introduce ConnectorDescriptor

Once a factory supports different SSH agents on the same platform,
which is planned for Windows once we use Apache MINA sshd 2.8.0,
client code may need to have a way to specify which SSH agent shall
be used when the SSH config doesn't define anything.

Add a mechanism by which a ConnectorFactory can tell what Connectors
it may provide. Client code can use this to set the identityAgent
parameter of ConnectorFactory.create() to the wanted default if it
would be null otherwise.

A ConnectorDescriptor is a pair of strings: an internal name, and a
display name. The latter is included because client code might want to
communicate agent names to the user, be it in error messages or in some
chooser dialog where a user could define which of several alternative
SSH agents should be used as default. The internal name is intended to
be used in the IdentityAgent directive in ~/.ssh/config.

Also make the ConnectorFactory discovered via the ServiceLoader
accessible and overrideable. Provide static get/setDefault() methods,
similar to the SshSessionFactory itself.

Change-Id: Ie3d077395d32dfddc72bc8627e92b23636938182
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>