feat: Add support for flexible defaults encoding

[ghostbuster91]

PR Checklist (not all items are relevant to all PRs)

• [x] Added unit-tests (for runtime code)
• [ ] ~Added bootstrapped code + smoke tests (when the rendering logic is modified)~ N/A
• [ ] ~Added build-plugins integration tests (when reflection loading is required at codegen-time)~ N/A
• [ ] ~Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)~ N/A
• [ ] ~Updated dynamic module to match generated-code behaviour~ N/A
• [x] Added documentation
• [x] Updated changelog

Refers to #1398

[ghostbuster91]

I am not happy about the implementation of SkipIfEmptyCollection so I am fine with dropping it at this stage to minimize the amount of changes, but wanted to showcase it in case you think that it is worth to have something like that.

[Baccata]

A good start 👍

[ghostbuster91]

@kubukoz wanna have a look?

[Baccata]

Good work. Just a couple nitpicks (and also this : https://github.com/disneystreaming/smithy4s/pull/1652#discussion_r1971927935)

[kubukoz]

@ghostbuster91 fyi if you said "Closes #1398" merging this would've closed the ticket, worth remembering in the future