feat(plan): add plan section support

[flotter]

Add support for Pebble Plan extensions to allow managers to extend the existing Pebble Plan schema.

// LayerSectionExtension allows the plan layer schema to be extended without
// adding centralised schema knowledge to the plan library.
type LayerSectionExtension interface {
	// ParseSection returns a newly allocated concrete type containing the
	// unmarshalled section content.
	ParseSection(data yaml.Node) (LayerSection, error)

	// CombineSections returns a newly allocated concrete type containing the
	// result of combining the supplied sections in order.
	CombineSections(sections ...LayerSection) (LayerSection, error)

	// ValidatePlan takes the complete plan as input, and allows the
	// extension to validate the plan. This can be used for cross section
	// dependency validation.
	ValidatePlan(plan *Plan) error
}

type LayerSection interface {
	// Ask the section to validate itself.
	Validate() error

	// Returns true if the section is empty.
	IsZero() bool
}