Add ko builder

[halvards]

Tracking issue for the effort to add the ko builder.

See the design proposal for further details.

Related: #5611, #1958

[halvards]

Related PRs in google/ko:

• Enable embedding of ko publish #348
• Add flag and PublishOption for destination repo #351
• Enable setting the working dir for the go tool #365
• Enable overriding docker client
• Add support for Go build flags #340
• Enable setting environment variables in .ko.yaml #419
• Set build config via BuildOptions #421

[tejal29]

Thanks for creating the issue!