[wasm-metadata] parse OCI author custom section

[yoshuawuyts]

This PR adds support for parsing the author custom section, implementing parts of #1922. Thanks!

Draft Notes

I'm filing this PR early and as a draft because I'm not 100% sure I'm following best practices here. It's my first time using wasmparser and wasm-encoder, and I'm for example not sure whether parse_wasm should take a BinaryReader or CustomSectionReader.

Also: I was looking for something like a Parse trait, but I don't think wasmparser has one? I quite liked the encoder side of things.

Finally: in this PR I'm basically just wrapping CustomSection in a newtype - however that means storing the internal data as a byte array rather than a string. How do we feel about that?

I'm asking these questions since I'm planning to repeat this pattern a number of times for the other custom sections, and it seems worth making sure it's early on. Thanks!

[alexcrichton]

Thanks for this! What I might recommend is to take the &[u8] entire wasm in memory as opposed to just a custom section. That way you can bake in how to handle a missing section, duplicated sections, the section name, etc. Internally you can use a BinaryReader to parse the binary data if necessary, but yeah the Parse-trait side of things is sort of there with FromReader but it's no the same as the encoding side of things.