Generate `Getter` trait boilerplate using existing proc macros

[Dav1dde]

Currently the Getter trait requires a bunch of manual boilerplate, we should be able to auto generate all of it using our existing derive macros, or at least make the top level implementation of Getter much much simpler, leveraging the IntoValue/FromValue/ProcessValue macros.

For example this PR exposes fields which could have just been auto-exposed and synchronized with the derive macros: https://github.com/getsentry/relay/pull/4711