feat: Basic attachment support

[timfish]

• Tested working against sentry.io
• Follows the other sdks by adding attachments to the scope

You can capture an event with a minidump attachment like:

    sentry::with_scope(
        |scope| {
            scope.add_attachment(Attachment {
                filename: "minidump.dmp".to_owned(),
                buffer: Vec::new(), // <-- minidump bytes go here
                ty: Some(AttachmentType::Minidump),
                content_type: None,
            })
        },
        || capture_event(Default::default()),
    );

[Swatinem]

lgtm. Too bad adding the content_type is technically a breaking change :-(

An idea that is also implemented in other SDKs is to have "lazy" attachments, where you just give a PathBuf and it will read the attachment lazily only when you really send it out. Not sure its worth changing the implementation right away though.

Some tests for this would be nice as well ;-)