chore: add typehint to ProcessorInterface

[alanpoulain]

[soyuka]

I'm in favor of mixed as it's definitely possible on an RPC route to use arrays, integers, strings, null or any data type in a POST operation according to the HTTP specification. If we use objects only we may end up using objects representing scalar values (for example NullContent or BinaryContent). Not sure there's a real benefit in doing so.

[stale[bot]]

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[soyuka]

we decided to keep mixed