Extend plugin API to support transfer handlers

[mandelsoft]

What this PR does / why we need it

The OCM library already provides an extension point to provide transfer handlers
controlling the transport of component versions from one environment into another one.

A transfer handler controls:

• how to handle changes in component versions when re-transporting them
• how to resolve component references and whether they should be transported, also
• whether sources and resources should be transported by value or by reference.

So far, there were two implementations:

• a standard implementation working on the standard transfer options, like --recursive or --copy-resources
• a spiff++ based implementation based on the standard handler adding the capability to influence
  the behaviour based on a spiff script.

On the command line interface always the spiff handler was used which works like the standard handler if no script
is configured.

This PR adds three new features:

• it supports a hierarchical naming scheme to denote transfer handlers (like for download and upload handlers),
  which can be used un the command line to select and configure supported transfer handler types.
• it extends the Plugin API to support transfer handler. This way a plugin may implement the transfer handler interface to
  provide an arbitrary rule set controlling the transport process.
• A general configuration option for handlers allows for passing additional handler configuration

Similar to the uploaders and downloaders the naming scheme for transfer handlers looks as follows:

• ocm/<builtin handlers>: used for the already existing standard and spiff handlers.
  Additional handlers may now add themselves to appropriate names to be usable from the command line.
• plugin/' A dedicated handler maps plugin provided transfer handlers into the naming scheme.
  It uses the sub scheme <plugin name>[/<handler name>]. A plugin may provide multiple (named) handlers.

Which issue(s) this PR fixes

[fabianburth]

Suggested change

	type testDH struct {
	DecisionHandlerBase
	}
	func (d *testDH) DecideOn(p Plugin, question interface{}) (bool, error) {
	panic("just a type test")
	}
	var _ DecisionHandler = (*testDH)(nil)

remove