Make pub web socket API follow JSON-RPC 2.0

[munificent]

Right now, it has an ad-hoc JSON structure. May as well follow a standard.

[munificent]

Marked this as blocking #15830.

[nex3]

Marked this as blocking #17200.

[nex3]

Set owner to @nex3.
Added Started label.

[nex3]

r34223 creates a package that implements a JSON-RPC 2.0 server.

[nex3]

r34269 converts pub's admin UI port's web socket API to use JSON-RPC 2.0.

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Added Fixed label.

[nex3]

Unmarked this as blocking #17200.