Add widestring support and improve Rust ABI parameter handling

[Branden Bonaby (bbonaby)]

Why is this change needed?

This PR introduces initial widestring support and refines Rust ABI code generation to better align with FlatBuffers semantics and idiomatic Rust borrowing patterns.

What changed?

• Added widestring as a workspace dependency and replaced the custom edl::WString wrapper with widestring::U16String / U16Str throughout generated Rust code.
• Updated generated function signatures to use borrowed forms where appropriate (e.g. Option<&T>, Option<&mut T>, string slices for in parameters).
• Introduced a shared assign_if_some helper to safely propagate optional out / inout values from ABI results.
• Simplified and centralized Rust type generation logic for:
  • Optional<T>
  • Vec<T> vs slices for in parameters
  • Arrays
  • String and wstring handling
  • Updated generated enclave and host stubs to correctly clone, copy, or borrow values based on directionality and type.
  • Added explicit conversion implementations between ABI WStringT and widestring::U16String.
  • Updated sample code to use &str instead of &String for string inputs and U16Str instead of &edl::WString for wstring inputs.

How was this tested?

• Confirmed both hello world, and userboundkey sample build and both run as expected without regressions.

Motivation

These changes:

• Rust developers expect slices and string slices for [in] values rather than borrowing owned types when they won't be modifying them.
• Reduce unnecessary allocations in generated Rust code. For [in] types in the past developers may have had to allocate an owned string (to_string() or vec type (to_vec) just to pass data to the codegen functions. Then in the codegen'd code we have to make a copy of them anyway when converting it to a flatbuffer type. Now we have one less allocation since the [in] will be a slice.
• Make ownership and mutability explicit at the API boundary. We now only pass mutable owned types for [inout] and [out] parameters.
• Lay the groundwork for more idiomatic and maintainable Rust codegen going forward