aya-obj: add several accessors for BTF types parsed by aya-obj

[Evian-Zhang]

I'm new to this project, and I want to use aya to extract BTF information in an ebpf object. I find aya extremely powerful in parsing ebpf objects, with aya_obj::Object::parse (by printing the parsed object, I'm sure that all BTF information is parsed). However, current API design makes it hardly to do anything with parsed BTF information. Almost all accessors are pub(crate). The only thing I can do is id_by_type_name_kind, which has far less information than aya actually got.

So in this PR, I make some pub(crate) APIs public, and add some new accessors.

Added/updated tests?

We strongly encourage you to add a test for your changes.

• Yes
• No, and this is why: the pub(crate) APIs are already tested previously
• I need help with writing tests

Checklist

• Rust code has been formatted with cargo +nightly fmt.
• All clippy lints have been fixed.
  You can find failing lints with cargo xtask clippy.
• Unit tests are passing locally with cargo test.
• The Integration tests are passing locally.
• I have blessed any API changes with cargo xtask public-api --bless.

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