Jib configuration block should allow explicitly setting type

[briandealwis]

With #2808 we merged the jibMaven and jibGradle blocks into a single jib block where we determine the plugin-type based on the presence of key files like build.gradle. This isn't fool-proof and we should allow setting the value explicitly.

[briandealwis]

Note: #2808 uses an internal enum to represent the plugin type:

skaffold/pkg/skaffold/jib/jib.go

Lines 45 to 52 in 4145d76

	// PluginType defines the different supported Jib plugins.
	type PluginType int
	const (
	// use `iota+1` so that 0 is an invalid value
	JibMaven PluginType = iota + 1
	JibGradle
	)

I had originally placed this enum definition inside the schema package, with appropriate MarshalYAML() and UnmarshalYAML() functions. This supported validation during the unmarshalling phase, but the schema generator didn't support these enums and so I backed it out.