Switch jsonschema validation libraries

[sudo-bmitch]

This swaps out https://github.com/xeipuuv/gojsonschema for https://github.com/santhosh-tekuri/jsonschema. Considering how far back some of this code goes, feedback from @stevvooe and @vbatts would be awesome.

[rchincha]

I would also keep a fork of this dep under OCI repos because the author may:

1. delete the repo or change access
2. change license
3. some other apocalyptic event ...

[sudo-bmitch]

I would also keep a fork of this dep under OCI repos because the author may:

1. delete the repo or change access

2. change license

3. some other apocalyptic event ...

I think most of those concerns are covered by individual developers with the go module cache on their machines, in addition to Google's Go proxy server: https://sum.golang.org/.