Add Jib detection to skaffold init

[TadCordle]

Review after #2327.

This PR adds Jib detection to skaffold init. Skaffold walks the directory searching for dockerfiles, build.gradles, and pom.xmls. If a build.gradle or a pom.xml is found, skaffold will attempt to run an internal Jib gradle task/maven goal to determine if Jib is configured on any of the projects (Jib v1.3.0 or higher is required for this to work).

If Jib is detected and jib.to.image is configured with an image found in the k8s manifest, the Jib builder is added to the skaffold config automatically.

If Jib is detected but jib.to.image is not configured to build a k8s image, the Jib project is added to the CLI prompt so the user can decide which image Jib should build, and the user's choice is added using the -Djib.to.image=... flag.

Issues left to solve:

• Configuring to build with Jib via CLI artifacts
• Gradle/Maven edge cases (config files that don't use the default names, unorthodox project structure, etc.)
• Generating Jib config automatically if it's missing?

Towards #1264 (probably not completely fixed until there's some idea how to automatically apply Jib to maven/gradle projects).

[codecov]

Codecov Report

Merging #2276 into master will increase coverage by 0.43%.
The diff coverage is 88.31%.

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/initializer/init.go	53.6% <81.81%> (+9.62%)	⬆️
pkg/skaffold/jib/jib_init.go	89.39% <89.39%> (ø)
pkg/skaffold/docker/docker_init.go	100% <0%> (+6.66%)	⬆️

[chanseokoh]

Looks good.