feat: make ChangeSet and KVPair protobuf serializable (backport #726)

[mergify]

This is an automatic backport of pull request #726 done by Mergify.
Cherry-pick of 61be28a has failed:

On branch mergify/bp/release/v0.20.x/pr-726
Your branch is up to date with 'origin/release/v0.20.x'.

You are currently cherry-picking commit 61be28a.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   buf.gen.yaml
	new file:   proto/buf.lock
	new file:   proto/buf.md
	new file:   proto/buf.yaml
	new file:   proto/changeset.pb.go
	new file:   proto/changeset.proto

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   CHANGELOG.md
	deleted by us:   diff.go
	deleted by us:   diff_test.go
	both modified:   nodedb.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[yihuang]

It seems the whole diff/changeset thing don't get backport-ed to v0.20.x, I guess we can open a separate PR to backport the whole thing if want to.
v0.20.x's changelog seems the same as v0.19.x, what's the plan for v0.20.x?

[tac0turtle]

.20 is what is used in the sdk right now. @cool-develope can create an issue or doc on whats needed for the migration? which versions and so on