kbld resolve as a Go library

[jpmcb]

Describe the problem/challenge you have

kbld is great at resolving images from a .imgpkg config and OCI bundle.

I want to harness that in my own program by using kbld's Go code. But currently, the only public methods available are the top level Cobra commands:

package main

import (
        goUi "github.com/cppforlife/go-cli-ui/ui"
        kbld "github.com/k14s/kbld/pkg/kbld/cmd"
)

...

        kbld.NewResolveCmd(kbld.NewResolveOptions(goUi.NewNoopUI()))

This isn't useful since the cobra commands Run can't be utilized as a function that returns a the resolved yaml chunk.

It is a non-goal to have to use os.Exec to call out to kbld. I don't want to have to require my users to have kbld on their system in order to use my tool. Instead, we want to compile and utilize the go code as a single binary.

Describe the solution you'd like

A Go API for utilizing kbld's image resolution functionatliy. This API should return the resolved yaml chunk, just like it does on the command line.

Anything else you would like to add:
This is in support of: vmware-tanzu/community-edition#3034

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