Bincompat-friendly codegen

[kubukoz]

Related: #1485, smithy-lang/smithy#2243, Discord discussion

In some situations, such as generating protocol shapes (not limited to, but in particular: traits), it would be beneficial to allow evolving models without breaking binary compatibility of generated code.

For example, adding a new field to a struct trait shouldn't break binary compatibility of the result.

We should come up with and agree on:

• conditions under which we would generate a shape as bincompat-friendly
  • for example: all traits? per-namespace/per-shape configuration? Something like what we're doing for validated newtypes in Validated newtypes #1454?
• an exact encoding that'll work across all supported Scala versions. Some starting points for discussion:
  • non-case classes
  • public getters
  • public withX (for a member named x) instead of copy methods
  • mutable builders?

and implement these for the next minor version (0.19), if possible.