Merge branch 'main' of https://github.com/randombitsco/swift-openai-bits

Set requests to use POST when they have a body.

* Fixed type for `Completions.Request.n`.

* Removed redundant `Equatable` conformance for `Identifier`
* Renamed `Utils` to `JSON`, improved parsing
* Added proper `Codeable` support for `Prompt`

Added missing initialisers.

Improved type handling when encoding/decoding

Improved JSON encode/decode

Reverted Swift 5.7 code to 5.5

Bumped Swift version back to 5.5

Bumped Swift version back to 5.5