[REQ] [RUST] Allow to choose different `Vec`, `String` and `HashMap` types

Is your feature request related to a problem? Please describe.

Unnecessary allocations.

Describe the solution you'd like

I'd like to use:

• smallvec instead of Vec
• either smartstring or smolstr instead of String
• heapless::FnvIndexMap or similar instead of HashMap

Describe alternatives you've considered

I tried using something like --import-mappings=Vec=smallvec::SmallVec --type-mappings=Vec=smallvec::SmallVec but it didn't work.

I also considered having a const array as input, but this wouldn't work for deserialization and would need to be wrapped into an enum with a variant for heap and a variant for the stack which would just do the same thing as smallvec, tinyvec etc.

Additional context

I haven't thought through how compatibility with serde would work.

No files or tests are named. Start by examining the Rust generator's handling of Vec, String, and HashMap mappings, then investigate compatibility with serde and the requested smallvec, smartstring, smolstr, or heapless types. Done means users can configure these alternatives and generated code still supports deserialization.