[bitcoin/bitcoin] Refactoring CRPCCommand with enum category (#13945)

> A drawback of this is that it centralizes knowledge of the command categories in the source code; right now, a module such as the wallet can register its own commands in its own category, without having to update a central file. This is a good form of decoupling.
>
> This is not as bad for categories as it would be for, say, commands themselves, but still.
@laanwj the target of this refactoring, is right to centralise the knowledge of the command categories, to avoid misuse of categories, like it was done for `hidden` category.

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