rustdoc-json: Structured attributes

Implements https://www.github.com/rust-lang/rust/issues/141358.

This has 2 primary benefits:

1. For rustdoc-json consumers, they no longer need to parse strings of
   attributes, but it's there in a structured and normalized way.
2. For rustc contributors, the output of HIR pretty printing is no
   longer a versioned thing in the output. People can work on
   https://github.com/rust-lang/rust/issues/131229 without needing to
   bump `FORMAT_VERSION`.

(Over time, as the attribute refractor continues, I expect we'll add new
things to `rustdoc_json_types::Attribute`. But this can be done
separately to the rustc changes).

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