[shared][rust] Add proc-macro to deserialize tagged enum variants

Summary:
Detailed description of why we need this: ENG-8901.

Unfortunately, proc-macros need to be defined in a separate crate (see why), so I created a util-macros crate in our Cargo workspace.

Test Plan:

• Added rustdoc usage example
• Rust unit tests added in the next diff
• Used and tested in in D12895
• Rust playground - see Linear description

Reviewers: varun, will, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12901