Add half feature with GetSize impls for f16 and bf16

[AlJohri]

Summary

• Add optional half Cargo feature exposing GetSize impls for half::f16 and half::bf16.
• Both types are 2-byte POD with no heap allocation, so the trait's default get_heap_size_with_tracker (returns 0) is sufficient — matches the pattern used for chrono::NaiveDate etc.

Cross-check

Mirrors the surface used by mem_dbg-rs (impl_size_of!(True; half::f16, half::bf16)): same two types, no SIMD wrappers (f16x4, etc.), heap size = 0.

Test plan

• cargo test -p get-size2 --features half test_half
• cargo build -p get-size2 --all-features