Refactor Gradle build config

[aSemy]

Initial attempt at #10841

I thought I'd make a PR to demonstrate what #10841 would end up looking like :). Some of this can be split up into smaller PRs

This covers most of the improvements in #10841

Summary

• use test-fixtures for sharing test utilities
• create convention plugins for sharing build config between subprojects
• set up dependencies catalog
• move subproject specific configuration into each subproject's build file
• move build properties into single extension, IntellijRustBuildProperties.kt
• add descriptions to subprojects
• use typesafe project accessors
• update sharing of Kotlin source files to use Configuration-based sharing
• create specific task, UpdateCargoOptions, for generating CargoOptions.kt (moving it out of the build script helps keep the build logic more organised, and creating a specific task makes it easier to use the Gradle API as intended)
• The IntelliJ platform specific properties have been moved into an enum class, which helps simplify the property files

todo

• the gradle-231.properties and gradle-232.properties files are ignored - I've got an idea of how to make this better...

Result

• The root build.gradle.kts is smaller and more manageable
• Convention plugins mean that build config is more easily shared
• most (all?) cross-project configuration has been removed
• dependencies can be defined in a single place: ./gradle/libs.versions.toml
• projects can share test utilities more easily, without also sharing test files

Changelog

changelog: refactored Gradle build config (no functional changes)