Add optional rust_decimal and indexmap feature flags

[WiggidyW]

Summary

• Add optional rust_decimal feature with ZodSchema impl for rust_decimal::Decimal (maps to z.number())
• Add optional indexmap feature with ZodSchema impl for indexmap::IndexMap<String, T> (maps to z.record(z.string(), T))

Both features are off by default and gated behind #[cfg(feature = "...")].

Usage

[dependencies]
zod_gen = { version = "1.3.0", features = ["rust_decimal", "indexmap"] }