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.

[kubukoz]

Made some progress with @Baccata.

I won't write down the exact encoding just yet (I changed my mind several times in the last couple hours), but it appears that we'll draw a lot of inspiration from Contraband, including a @since("1.0") trait (or similar) to allow marking new members as having been added in a particular version.

If there's a bunch of versions, we can generate a constructor / apply for each of them, e.g.

//assume everything is required

structure Foo {
  s1: String
  @since("1.0") s2: String = "test2"
  @since("1.0") s3: String = "test3"
  s4: String
}

// we can add these traits for external shapes too, with `apply`
apply Foo$s4 @since("2.0")
apply Foo$s4 @default("test4")

final case class Foo private(s1: String, s2: String, s3: String, s4: String) { ... }
object Foo {
  def apply(s1: String): Foo = new Foo(s1, "test2", "test3", "test4")
  def apply(s1: String, s2: String, s3: String): Foo = new Foo(s1, s2, s3, "test4")
  def apply(s1: String, s2: String, s3: String, s4: String) = new Foo(s1, s2, s3, s4)
}

I'll play with Contraband soon to see if it does this (and other interesting cases it handles, such as adding new values between existing ones) and get back with an update.