Implement templates

[bprashanth]

Tracks the implementation of the templating proposal: https://github.com/kubernetes/kubernetes/blob/master/docs/proposals/templates.md.

This wip is essentially a quick merge of os templates with HEAD + some modifications to get the core substitution logic and resource to work with apiserver: #23895

See proposal for details on what remains, from memory:

• (()) syntax
• /processedTemplates and /templates api endpoints
• pkg/client
• kubectl integration
• validation
• Unittests for templates, etcd etc
• e2es
• docs

Other items to track

• Template processing
• Template create validation + etcd_test.go tests
• Template update validation + etcd_test.go tests
• Processing templates without saving them

@bgrant0607 mentioned someone from @kubernetes/kubectl might be interested.
@bparees since you had the original proposal.

[bgrant0607]

cc @pwittrock @janetkuo

[0xmichalis]

cc @mfojtik

[soltysh]

/cc @bparees

[bgrant0607]

An implementation in Rust:
https://github.com/InQuicker/ktmpl

[bgrant0607]

Kubecon presentation:
https://www.youtube.com/watch?v=__ic1Q94osA&index=28&list=PLosInM-8doqcBy3BirmLM4S_pmox6qTw3

[bgrant0607]

ref #25293, #25622