Fix Rust codegen: use manual impl Default for struct object types

Problem

When a FlatBuffers struct contains a fixed-size array with more than 32 elements (e.g. [ubyte:64]), the generated Rust object API type uses #[derive(Default)] which fails to compile on Rust < 1.61, because Default was not implemented for [T; N] where N > 32 until that version.

Fix

Changed the Rust code generator (src/idl_gen_rust.cpp) to emit a manual impl Default for struct object types instead of
#[derive(Default)]. This matches the pattern already used for table object types.

The generated default values are:

• Scalar/enum fields: use GetDefaultValue() (0, false, etc.)
• Arrays of builtins: [0; N] (avoids the Default trait bound)
• Arrays of structs/enums: ::flatbuffers::array_init(|_| Default::default())

Code generation changes

Changes were made to src/idl_gen_rust.cpp.

Test

Added LargeArrayStruct { d:[ubyte:64]; } to tests/arrays_test.fbs .

Closes #8943