client: offer a json backend for the state store

[schmichael]

Description

Inspired by @pkazmierczak's work in #27934 and Tailscale, I wanted to explore the "simplest" possible client state solution that was still reasonably safe. So here's a json backend for our client state that has to perform like complete garbage. If only I had written the bolt de-duplication layer in c0d1b63 generically so it could be reused here. 🤷

Testing & Reproduction steps

No tests. Do Not merge.

Should probably fsync the directory itself to ensure crash safety like raft-wal does: https://github.com/hashicorp/raft-wal/blob/f9846d205596489efa6a2a04d72a564a7fecbe44/fs/fs.go#L117-L128

Easiest way to test is to edit this line to return NewJsonDB:

nomad/client/state/db_bolt.go

Line 173 in 83ef3d0

return NewBoltStateDB

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Changes to Security Controls

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