Support Check-And-Set deletion of config entries

[boxofrad]

Adds support for Check-And-Set (CAS) deletion of config entries, similar to that of KVs.

This is useful for cases where it is desirable to delete a config entry, but only if it has not been modified by another process since reading it (such as the scenario described in #11372).

Sidenote: this is my first PR to Consul 🎉 so any meta/style feedback is greatly appreciated!

[hashicorp-cla]

All committers have signed the CLA.

[hashicorp-ci]

🤔 This PR has changes in the website/ directory but does not have a type/docs-cherrypick label. If the changes are for the next version, this can be ignored. If they are updates to current docs, attach the label to auto cherrypick to the stable-website branch after merging.

[freddygv]

Awesome work with your first PR! Thanks for tackling this gap.

I left some comments/questions below.

[freddygv]

Is this backwards compatible with older clients?

[boxofrad]

I think so, yeah. Older clients will unmarshal the response into a struct{} which seems to be fine:

package compattest

import (
	"testing"
	"bytes"

	"github.com/hashicorp/go-msgpack/codec"

	"github.com/hashicorp/consul/agent/structs"
)

func TestBackwardCompatibility(t *testing.T) {
	var b bytes.Buffer
	encoder := codec.NewEncoder(&b, structs.MsgpackHandle)
	decoder := codec.NewDecoder(&b, structs.MsgpackHandle)

	if err := encoder.Encode(&structs.ConfigEntryDeleteResponse{Deleted: true}); err != nil {
		t.Fatal(err)
	}

	var foo struct{}
	if err := decoder.Decode(&foo); err != nil {
		t.Fatal(err)
	}

	var bar string
	if err := decoder.Decode(&bar); err == nil {
		t.Fatal("should not have been able to decode into a string")
	}
}

[freddygv]

Cool, I also tried it out locally with two agents on different versions and it worked

[freddygv]

🚀

[hc-github-team-consul-core]

🍒 If backport labels were added before merging, cherry-picking will start automatically.

To retroactively trigger a backport after merging, add backport labels and re-run https://circleci.com/gh/hashicorp/consul/490134.