build: migrate to use buildSrc and convention plugins

[dariuszkuc]

📝 Description

Additional information:

• build sources
• convention plugins

🔗 Related Issues

[eg-oss-ci]

Apollo Federation Subgraph Compatibility Results

Federation 1 Support	Federation 2 Support
_service 🟢 @key (single) 🟢 @key (multi) 🟢 @key (composite) 🟢 repeatable @key 🟢 @requires 🟢 @provides 🟢 federated tracing 🟢	@link 🟢 @shareable 🟢 @tag 🟢 @override 🟢 @inaccessible 🟢

Learn more:

• Apollo Federation Subgraph Specification
• Compatibility Tests

[dariuszkuc]

TLDR this moves subproject build logic from root build.gradle.kts files to buildSrc and makes it a convention plugin that is applied to individual projects.

There are few outstanding Gradle issues

• Version catalog accessors for plugin aliases shown as errors in IDE kotlin script editor gradle/gradle#22797 - we need to suppress DSL_SCOPE_VIOLATION when referencing version catalog in plugins block
• Make generated type-safe version catalogs accessors accessible from precompiled script plugins gradle/gradle#15383 - we need a workaround to access version catalog in convention plugin