[C#] Change class to readonly struct for records

[andreakarasho]

By design the record wit type is just a set of fields (https://component-model.bytecodealliance.org/design/wit.html#records) This PR aims to reduce the GC work changing from a class to a readonly struct.

[GerardSmit]

Additional: the design document states the following

Records are similar to a struct in many languages.
https://github.com/WebAssembly/component-model/blob/2c17516179f99accdafb01d8e4affcc0f58184cc/design/mvp/WIT.md?plain=1#L1514-L1516

It's also not possible to recursively reference a record. wit-bindgen already checks for this. I tried to reference point in point:

   0: failed to parse package: wit
   1: type `point` depends on itself

https://github.com/bytecodealliance/wasm-tools/blob/544ac54d0d9d43bcfed2597af4293729dfb89854/crates/wit-parser/src/ast/toposort.rs#L169-L171

[yowl]

Thanks, I need to fix the CI first.