Allow configuring jib plugin type

[briandealwis]

Fixes #2963.

Note that this is a somewhat unorthodox change as it exposes an internal and previously ignored field from the v1beta14's jib config.

Description

In #2808 we unified the jibMaven and jibGradle configuration blocks into a single jib block. The Skaffold-Jib builder looks for certain files to determine the Jib plugin type (Maven or Gradle). We've had a report where Skaffold-Jib was unable to guess the Jib plugin type. This PR adds the ability to explicitly configure the jib block to specify the plugin type.

The unified JibArtifact already has a Type field which allows explicitly specifying the builder type. This field is internal and not marshalled. The intent was to expose this field to encode a Go enum (with corresponding MarshalYAML() and UnmarshalYAML()) but I punted at the time as it required teaching the schema generator to output JSON schema enum. But I realized that the pkg/skaffold/schema/validation approach was easier, but it requires changing the type of Type.

User facing changes
This PR changes the type of JibArtifact's Type field and exposes it. As a result:

• v1beta14 and v1beta15's jib now have optional type fields
• skaffold fix on v1beta13 jibMaven and jibGradle configurations will now have a type field

Reviewer Notes

• The code flow looks good.
• Unit test added.
• User facing changes look good.

Release Notes

jib artifacts can explicitly specify the plugin type as maven or gradle

[chanseokoh]

Is it possible to update old schemas? There's a build error saying you shouldn't (if I understood it right).

[codecov]

Codecov Report

Merging #2964 into master will increase coverage by 0.04%.
The diff coverage is 93.75%.

Impacted Files	Coverage Δ
pkg/skaffold/schema/v1beta14/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/latest/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta15/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta16/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta13/upgrade.go	92.3% <100%> (+0.64%)	⬆️
pkg/skaffold/schema/validation/validation.go	100% <100%> (ø)	⬆️
pkg/skaffold/jib/jib_init.go	89.55% <100%> (+0.66%)	⬆️
pkg/skaffold/jib/jib.go	73.48% <50%> (ø)	⬆️

[balopat]

This should be fine as it actually helps making the upgrade process to not break when the detection mechanism doesn't work. It is a conservative change. I will use admin rights to merge this PR.

[container-tools-bot]

Please visit http://35.236.103.231:1313 to view changes to the docs.

[chanseokoh]

The Jib logic part looks good.

[briandealwis]

Sorry @balopat, this totally slipped through the cracks.

[balopat]

I wonder if we can make the string an enum / oneOf somehow

[balopat]

Suggested change

	// Type the Jib builder type; normally determined automatically. Valid types are:
	// Type the Jib builder type; if not set then it is detected automatically. Valid types are:

[briandealwis]

I need to update this in light of #3041

[briandealwis]

I hacked up the schema generator to emit JSON schema enums. PTAL!

[balopat]

This is awesome, thanks for putting the enum together as well!

[balopat]

Merged with admin privileges for the historical version change as it actually helps with upgrades.