THRIFT-6059: add crate_prefix option to Rust codegen for cross-file imports

[santiagomed]

Problem

The Rust code generator hardcodes use crate:: for cross-file imports (e.g., fileA.thrift includes fileB.thrift generates use crate::fileB;). This assumes the generated files are placed at the crate root (src/). When files are placed in a submodule (e.g., src/thrift/), the correct prefix is super::, and users must post-process generated files with sed.

Fix

Add a crate_prefix generator option that controls the path prefix. Default remains crate for backward compatibility.

# Files at crate root — existing behavior, unchanged
thrift --gen rs -out src/ schema.thrift

# Files in a submodule
thrift --gen rs:crate_prefix=super -out src/thrift/ schema.thrift

Changes

• t_rs_generator constructor reads crate_prefix from parsed_options
• crate_prefix_ member stores the value (default: "crate")
• Import generation in render_program_includes uses crate_prefix_ instead of hardcoded "crate"
• THRIFT_REGISTER_GENERATOR updated to document the option

[santiagomed]

@mhlakhani @Jens-G could I get a review here please? Also open to hearing other solutions to this issue with submodules!