Refactor zbus_macros and zvariant_derive to use common code.

I've noticed there are similarities between what zvariant_derive and zbus_macros crates do. In particular both crates:

1. Parse structure/enum/field attributes. zvariant_derive was recently refactored to use cleaner code in !639 (merged), but zbus_macros has been left without any attention.
2. Convert cases. Case conversion code for zvariant_derive was copied with little changes from zbus_macros.

I suggest adding a new crate that contains utilities used by both crates.

Benefits:

1. Cleaner code that is easier to understand in both crates.
2. Less code duplication and faster compile times.
3. Easier changes to supported attributes in zbus_macros.

Drawbacks:

1. A new dependency for these crates. This dependency is fully controlled though so I don't think this is an issue.

Questions:

• What will the name of the crate be?
• Are there any requirements on the architecture for such a crate?

I can submit a MR in case you like the idea.